III-Nitride Based Avalanche Photo Detectors

* [email protected]; phone 1 847 491 7251; fax 1 847 467 1817; cqd.eecs.northwestern.edu

III-Nitride Based Avalanche Photo Detectors Ryan McClintocka, Erdem Ciceka, Zahra Vashaeia, Can Bayrama, Manijeh Razeghi*a, and Melville

P. Ulmerb

aCenter for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL USA 60208

bDepartment of Physics and Astronomy, Northwestern University, Evanston, IL USA 60208

ABSTRACT Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing III-Nitride photodetectors that utilize low-noise impact ionization high internal gain can be realized—GaN APDs operating in Geiger mode can achieve gains exceeding 1×107. Thus with careful design, it becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride visible-blind APDs and discuss the critical design choices necessary to achieve high performance Geiger mode devices. Other major technical issues associated with the realization of visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined. The photon detection efficiency, dark count rate, and spectral response of or most recent Geiger-mode GaN APDs on free-standing GaN substrates are studied under low photon fluxes, with single photon detection capabilities being demonstrated. We also present our latest results regarding linear mode gain uniformity: the study of gain uniformity helps reveal the spatial origins of gain so that we can better understand the role of defects. Keywords: APD, GaN, Geiger Mode, Avalanche Photodiode, Single Photon Detection

1. INTRODUCTION Humans are capable of detecting light with the millions of rod and cone cells that are distributed along the surface of the human retina. This biological detection system is only sensitive to a very small portion of the entire electromagnetic spectrum from about 400 to 750 nm. Outside of this narrow spectral range humans must rely on technology for light detection. Many novel technological approaches have been developed to address photodetection at wavelengths ranging from extremely short cosmic rays all the way to very long terahertz wavelengths. The bulk of these technologies operate by modulating the resistance of the device or by creating a photo-voltage or photo-current where the externally measured electrical signal is proportional to the incident photon flux. This type of technology works quite well for large signals; but light is ultimately quantized into individual photons, and as the photon flux approaches levels where single photons begin to be important, the output signal of these detectors becomes lost in the noise floor. In contrast to conventional photodetectors, the human eye was discovered to be capable of detecting a flash of light consisting of as few as 100 photons.1 Because this light is dispersed over many individual rod cells this suggested in the early 1900’s that human rod cells are capable of detecting single photons. Recent experiments have been able to quantitatively measure this signal and determine that rod cells are capable of detecting single photons with a probability approaching 30%.2 However, few man-made technologies are capable of matching this fidelity. Developing detectors that are capable to single photons has attracted the attention of scientists since the discovery that light was quantized. Many applications such as astronomy, Raman spectroscopy, and fluorescence spectroscopy, would benefit from detectors with such level of sensitivity. The recent development of quantum computing research and quantum key distribution relies on the ability to reliably detect single photons. Thanks to their high internal gain, photomultiplier tubes (PMTs) were the first devices to demonstrate single-photon counting capabilities. By changing the photocathode material PMTs can cover a wide spectral range from ultraviolet to infrared.

Invited Paper

Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon, edited by Eustace L. Dereniak, John P. Hartke,Paul D. LeVan, Randolph E. Longshore, Ashok K. Sood, Manijeh Razeghi, Rengarajan Sudharsanan,

Proc. of SPIE Vol. 7780 · 77801B · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.863962

Proc. of SPIE Vol. 7780 77801B-1

Downloaded from SPIE Digital Library on 21 Sep 2010 to 129.105.215.146. Terms of Use: http://spiedl.org/terms

Single photon detection in the ultraviolet (UV) is particularly important for a number of applications. Using a visible- or solar-blind detector eliminates the need for expensive and efficiency-limiting optical filters to remove undesired out-of-band visible or solar photons. This makes them well suited for numerous applications in the defense, commercial, and scientific arenas including covert space-to-space communications, early missile threat detection, chemical and biological threat detection and spectroscopy, flame detection and monitoring, UV environmental monitoring, and UV astronomy.,5,3,4

2. UV SINGLE PHOTON DETECTION III-nitrides are one of the few semiconductor materials with a wide enough band-gap to offer visible blind UV detection. III-nitrides have been studied extensively for use in ultraviolet (UV) photodetectors due to their potential to offer intrinsic visible- or solar-blind detection, which is highly desirable for a number of applications.5 However, despite the advantages of III-Nitride based UV photodetectors, many of these applications are still dominated by the use of photomultiplier tubes (PMT). PMTs are capable of obtaining very high sensitivity using internal electron multiplication gain (typically ~106), and with proper choice of the photocathode material it is also possible to obtain a degree of visible- or solar-blindness.6 III-Nitride based UV photodetectors can achieve similar sensitivity by taking advantage of avalanche multiplication under high electric fields – typically 2.7 MV/cm.7 Great strides have already been made in the realization of proof-of-concept UV avalanche photodiodes8,9,10,11,12 based on III-Nitrides. However, most of the early GaN based APD devices were designed for front-illumination. This configuration makes is possible to use free-standing GaN substrates or thick GaN templates on sapphire to achieve low dislocation density devices. Despite the relative ease of growing front-illuminated GaN APDs, the realization of back-illuminated GaN APDs has been demonstrated to be more advantageous.13 A back-illuminated p-i-n GaN structure, in which photons reach the n-type layer first, results in primarily the injection of holes into the high-gain multiplication region. This results in higher gain and better overall performance since, near the critical field strength, the hole impact ionization coefficient is higher than the electron impact ionization coefficient in GaN.14 In addition, the integration of APD into APD arrays with read-out electronics becomes easier. In addition to realizing high gain in linear mode operation, there are many applications that benefit from or even require single-photon detection capabilities from UV detectors. In APDs this can be accomplished through the use of Geiger mode operation. This entails the operation of an avalanche photodiodes well above the breakdown voltage in combination with pulsed quenching circuitry. When APDs are biased for Geiger-mode, a single photon with a sufficient energy can trigger a self-sustaining electron-hole multiplication process.31 Photon(s) incident upon the device are absorbed in the active volume of the avalanche photodiode while the device is negatively biased over the breakdown voltage. Through consecutive impact ionization events, the initial charge is amplified raising the output external current above the noise floor. This current pulse is detected by the electronics as a detection event. The APD is then quenched to stop the multiplication process. In a few nanoseconds, the flowing current becomes negligible, and the device is again biased over the breakdown voltage waiting for the arrival of a new photon. Instead of detecting the incident signal as an analogue current, the electronics digitally count the discrete photon absorption events in order to measure the signal. A discriminator circuit is used to detect the discrete pulses and differentiate from thermally generated pulses and other noise. The discriminator uses a comparator to evaluate the pulse height relative to a reference voltage. Pulses which exceed the discriminator threshold are counted, while those that do not are ignored. Hence, the discriminator reference voltage must be carefully adjusted in order to maximize the signal-to-noise ratio, i.e. the number of detected pulses over the number of undesirable counts. These undesirable counts are the result of the spurious emission of trapped charges that trigger the avalanche in absence of photons. They are called dark counts and must be minimized through material growth, device fabrication and optimum selection of operation parameters.

3. APD DESIGN CONSIDERATIONS The design of a high performance GaN based avalanche photodiode is requires taking into consideration the carrier multiplication dynamics, and the design of both the absorbing region and the multiplication region.



3.1. Hole vs. Electron Multiplication One of the most important first steps in designing an avalanche photo diode is to study the carrier multiplication dynamics. The multiplication is determined by the device design and geometry and the impaction ionization coefficients of the material. Electrons and holes will each have their own impact ionization coefficients (βp for holes, and αn for electrons), and these coefficients will vary with the applied electric field. In most practical semiconductors both the electrons and holes will contribute to the multiplication. The contribution of both carriers is required to see a sharp break-down characteristic: otherwise the multiplications will merely increase exponentially as: 15

Equation 1 ( )WdxM n

W

nn αα expexp0

=⎟⎠⎞⎜

⎝⎛= ∫

where Mn is the electron multiplication, αn is the electron impact ionization coefficients, and W is the thickness of the multiplication region. In the case where both carriers contribute to avalanche breakdown, the ratio of the ionization coefficients (βp/αn) determines the multiplication gain. Assuming electron multiplication dominates and the best case scenario that electrons are injected from the p-region (at the periphery of the high field region) and that the electric field is constant, the electron multiplication (Mn) can be modeled as: 15

Equation 2 [ ] [ ]{ }

[ ]{ })/(1exp)/(1

)/(1exp)/(1

npnnp

npnnpn W

WM

αβααβαβααβ

−−−−

=

where Mn is the electron multiplication, αn and βp are the electron and hole impact ionization coefficients, respectively, and W is the thickness of the multiplication region. From this equation we see that from the standpoint of achieving maximum linear mode gain it is advantageous to have similar ionization coefficients. However, this equation assumes that electron multiplication dominates. There is no guarantee that this is the case, in fact, Mote-Carlo simulation of III-Nitrides suggested that holes would have a higher impact ionization coefficient at practical electric fields.16 In the case where hole impact ionization dominates it is advantageous to inject holes from the n-side of the junction. This avoids the need to undergo electron impact ionization to generate free holes in the multiplication region and allows for a faster build-up of the carrier multiplication. In order to evaluate the relative role hole and electron impact ionization play in GaN APDs we grew and processed a GaN p-i-n homo-junction structure that could be illuminated either from the top of the bottom.17 The p- and n- region thicknesses were chosen to absorb 90% of the light. This results in minority carrier electron injection if the p-type GaN is illuminated, and minority carrier hole injection if the n-type GaN is illuminated.

0 20 40 60 80 100

1

10

100

Gai

n

Reverse Bias (V)

Mp (Back)

Mn (Front)

20 25 30 35 40 45 50 55101

102

103

104

105 βp (theory)

αn (theory)

αn

βp

Ion

izat

ion

co

effi

cien

t (c

m-1)

1/E (x10-4 cm/V)

Figure 1. Left) Multiplication factors for electrons (Mn) and holes (Mp) obtained from front and back illumination, respectively. Right) the impaction ionization coefficients for electrons (αn) and holes ((βp) extracted from the multiplication factors on the left. The theory was taken from REF. 16.



Photocurrent measurements were performed under back- and front-illumination with a frequency-doubled argon ion laser at 244nm (optical power=102 W/cm2). Multiplication factors were then calculated from these measurements. As observed in Figure 1(left), back-illumination provides higher gain than front-illumination. Ionization events start at voltages above 50V for holes and 70V for electrons in sample A. This agrees with the higher hole ionization coefficient that theory predicts, and which points to hole-initiated multiplication as a more beneficial concept in GaN.16 The ionization coefficients for holes (βp) and electrons (αn) can be extracted from the multiplication factors for electrons (Mn) and holes (Mp) as

Equation 3

( ) 1 ( )1( )

( ) ( ) ( )

( ) 1 ( )1( )

( ) ( ) ( )

p pp

p n n

n nn

n p p

M V M VE Ln

W M V M V M V

M V M VE Ln

W M V M V M V

β

α

⎛ ⎞− ⎛ ⎞= ⎜ ⎟ ⎜ ⎟⎜ ⎟− ⎝ ⎠⎝ ⎠

⎛ ⎞ ⎛ ⎞−= ⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟−⎝ ⎠ ⎝ ⎠

where E is the electric field, W is the multiplication region thickness, Mn and Mp are the experimentally measured electron (top illumination) and hole (bottom illumination) multiplication, and V is the applied reverse bias. This assumes that the electric field (E) is constant across the i-region (W) and equal to E=V/W. The resulting ionization coefficients are shown in Figure 1(right) compared to the theory of REF. 16. The higher multiplication gain for hole-injection is due to the larger impact ionization coefficient for holes in III-Nitrides. Because of magnesium memory effects is it not practical to grow n-on-p III-Nitride device. This forces the adoption of a back illuminated design in order to realize high gain GaN APDs. This also has the side effect of precluding growth on low dislocation templates like lateral epitaxial overgrowth (LEO), thick hydride vapor phase epitaxy (HVPE) GaN, or free-standing GaN substrates. Instead it is necessary to adopt a design that is based on UV transparent sapphire substrates and AlGaN or AlN template layers.

3.2. n-GaN absorber region thickness The bottom n-GaN layer is responsible for absorbing photons and injecting holes into the multiplication region. It also serves to buffer the high field multiplication region from the AlN template layer. The interface between the GaN APD and the AlN template is not lattice matched, and this interface contains a large number of defects. These defects increase the recombination rate at this interface which degrades the quantum efficiency of the device. If a high electric field is applied to this defected interface these defects can act as micro-plasma break-down centers.18 Thus it is necessary to shift the depletion region away from this interface by controlling the thickness of the bottom n-GaN layer.

+

-

+

ν

AlN

h

Defected

region

Depletion

region

i-GaN

n-GaN

Dif.

250 275 300 325 350 3750.01

0.1

1

10

100

300 nm 200 nm 100 nm

Qu

antu

m E

ffic

ien

cy (

%)

Wavelength (nm)

Figure 2. Left) Schematic band diagram of the device bottom layers and interface with the AlN template. Middle) Electric field build up and relative absorption showing where carriers are absorbed for different n-GaN layer thicknesses (100, 200 and 300 nm). Right) External quantum efficiency vs. wavelength for three different n-GaN layer thicknesses (100, 200 and 300 nm).



The thickness of the bottom n-GaN layer is critical to obtaining optimum performance. This layer separates the depletion region from the interface (Figure 2, left), so a minimum thickness is needed in order to avoid the interface defects. However, the n-GaN layer in a homojunction GaN APD is also responsible for absorbing a portion of the incident light (Figure 2, middle). As the absorbing layer is made thicker, more and more carriers are generated further and further from the multiplication region. If the thickness is too large holes may not reach the multiplication region due to the short diffusion length and other carrier collection issues. The right of Figure 2 shows the external quantum efficiency for identical devices with different n-GaN thickness of 100, 200, and 300 nm. The 100 nm device shows low quantum efficiency compared to the 200 nm device. The 300 nm device shows similar quantum efficiency to the 200 nm device near the band edge, but the quantum efficiency is significantly lower for shorter wavelengths. The optimum response is thus obtained for layer thicknesses of about 200 nm, which yield quantum efficiencies between 20% and 36%.

3.3. Active region thickness The thickness of the unintentionally doped (i-GaN) multiplication region also plays a critical role in the performance of the device. As shown in Equation 1 and Equation 2, the total multiplication is dependant on the multiplication width (W). As the multiplication width is increased, the carriers can travel farther before reaching the opposite end of the junction and thus can contribute to more multiplication events. However, increasing the multiplication width does not necessarily result in increased device performance. The ionization coefficients depend upon the electric field, which in turn depends upon the multiplication width and the resistance of the various constituent layers in the device. In a simple p-i-n avalanche photodiode, the insertion of an intrinsic i-GaN region between the p- and the n-GaN layers create a uniform electric field profile in which the carriers can be multiplied at a constant rate. The thicker the intrinsic region, the lower is the electric field strength for a given applied voltage, as shown on the left side of Figure 3. As the intrinsic region is made thinner the electric field increases, but only up to a point since the resistance of the n- and p-GaN layers is non-zero and a portion of the electric field always lies outside of the intrinsic multiplication region.

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

6

7

Ele

ctric

fiel

d (|

MV

/cm

|)

Depth (μm)

0 nm 50 nm 100 nm 150 nm 200 nm 250 nm 300 nm 350 nm 400 nm 450 nm 500 nm

0 100 200 300 400 5000

1

2

3

4

5

6

7

Critical Electricfield ~2.8 MV/cm

Ele

ctric

Fie

ld (

|MV

/cm

|)

i-GaN width (nm)

Figure 3 Left) Electric field profile of p-i-n GaN diodes under a constant applied bias of −80 V, with different intrinsic region thickness. Left) Plot of the peak electric field as a function of the intrinsic layer thickness. The horizontal line (red) indicates the experimentally determined critical electric field strength in GaN of ~2.8 MV/cm.13,

In order for the APD to undergo Geiger mode breakdown it is necessary for the electric field in the multiplication region to exceed the critical electric field strength. Previous experiments in GaN APDs have shown this value to be about 2.8 MV/cm.13,19,20 Ideally, the voltage could simply be increased as the intrinsic region is made thicker in order to maintain a constant electric field above the critical field strength. However the leakage current of the diode also increases as the applied reverse bias is increased. We have experimentally found 80–100 V to be a good operating point for APDs. At this voltage the leakage current ranges from 100 nA to 1 μA. The horizontal red line on the left of



Figure 3 shows a critical electric field strength of 2.8 MV/cm and the black line plots the peak electric field at a reverse bias of 80 V for various thicknesses of the intrinsic region. This suggests that the intrinsic region should be kept between 200 and 250 nm thick.

3.4. Separate Absorption & Multiplication In a normal back illuminated p-i-n device structure the absorption of photons occurs primarily in the n-type GaN layer, as was discussed in section 3.2, although a small portion of the photons are also absorbed in the multiplication region itself. The optimization of the bottom n-GaN absorption layer in a p-i-n APD is dominated by the need to avoid interfacial defects at the AlN interface, and the need to ensure holes generated in the n-GaN reach the multiplication region. One way to allow greater freedom in the design of the absorption region is to decouple the absorption from the multiplication by adopting a separate absorption and multiplication APD (SAM-APD) design. The use of separate absorption and multiplication (SAM) regions in avalanche photodiodes is a common approach to reduce multiplication noise and enhance gain through impact-ionization engineering.21,22,23

n-contactn-GaN

Sapphire (0001)

AlN (500nm)

LT- AlN buffer

Passivation

p-contact

i-GaN

i-GaN

p-GaN

n-GaN

λ

Multiplication

Absorption

Figure 4. Left) a conventional p-i-n APD structure. Right) the separate absorption and multiplication device structure.

The SAM-APD uses a p-i-n-i-n structure as shown on the right of Figure 4. Based upon this structure, the electric field profile can be calculated from a one-dimensional finite element model.24 This is shown below in Figure 5. In the SAM-APD structure light absorption in the bottom-most n-GaN and i-GaN layers combined account for 99% of the absorption. This results in nearly pure hole-injection into the multiplication region. However, the absorption properties change for photon energies above or below the bandgap25: as the photon energy increases above the bandgap, the light becomes mainly absorbed in the bottom n-GaN, closer to the AlN interface; contrarily, as the energy decreases below the bandgap, the light becomes mainly absorbed in the upper layers.26 In particular, the Franz-Keldysh effect may raise the absorption coefficient in the multiplication region at longer wavelengths.27 At low voltages, the electric field is mainly localized in the topmost i-GaN layer (multiplication region). The increase of the reverse bias enhances the electric field in this layer and broadens the depletion region towards the p-GaN and across the n-GaN layer towards the other i-GaN layer. This intrinsic layer becomes fully depleted at the reach-through voltage (Vrt = 40 V). Above this voltage, an electric field begins to appear in the i-GaN absorption region. This electric field drives a drift that assist with the injection of holes into the multiplication region and makes is possible to increase the i-GaN region thickness. The electric field and the thin n-GaN barrier between the two i-GaN regions also ensure that only holes initiate multiplication in the active region. This hole initiated multiplication results in lower excess noise in linear mode,13 but is not critical for Geiger mode operation.

Back Illuminated

p - GaN:Mg (285 nm)

Ti/Au n - GaN (200 nm)

Sapphire (0001)

AlN (500nm)

i -

LT - AlN buffer

p-contact

n-contact n-GaN

Sapphire (0001)

LT - AlN buffer

Passivation

n-GaN i-GaN

p-GaN

AlN Template



0.0 0.2 0.4 0.6 0.80.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 100 V 90 V 80 V 70 V 60 V 50 V 40 V 30 V 20 V 10 V 0 V

Ele

ctri

c fie

ld (M

V/c

m)

Depth (µm)

0.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e A

bsor

ptio

n

Multiplicationregion

Absorptionregion

p-GaN i-GaN n i-GaN n-GaN

Figure 5. Electric field profile and relative photon absorption profile of a SAM-APD showing most of the absorption taking place before photons reach the multiplication region.

The improved performance of the SAM-APD design has been confirmed for linear mode operation. Conventional and SAM-APDs were grown with similar doping and the same multiplication layer thicknesses. The conventional APD showed a maximum Gain of 5,700, where as the SAM-APD showed a significantly higher maximum gain of 41,200 (Figure 6).28 In addition to the higher gain, 1/f noise measurements of both APDs showed significantly lower noise for the SAM-APD, as is expected for single carrier initiated multiplication.

0 20 40 60 80 10010-1

100

101

102

103

104

105

Conventional p-i-n APD SAM-APD

Gai

n

Reverse Bias (V)

Figure 6. Comparison of the gain of conventional p-i-n APDs and SAM-APDs.

Although the gain of the SAM-APD is significantly higher in linear mode operation attempts to measure single photon detection from SAM-APDs have not proved successful. The SAM-APD design is ideal for realizing high linear mode gains; however, the addition of the n-GaN and i-GaN layers increases the breakdown voltage and increases the series resistance of the device. The higher breakdown voltage is due to the voltage drop across the first i-GaN layer that leads to a smaller electric field in the multiplication region. This means that it is necessary to apply more bias to the device in order to reach the critical field strength. This has the side effect of increasing the dark current at the onset of breakdown. This higher breakdown contributes to additional dark counts that mask the single photon detection signal.



The higher series resistance coupled with the lack of electrons being generated in the multiplication regions results in a more sluggish breakdown. To overcome this, a larger AC pulse must be applied to the APD which further increases the dark current and results in an increased dark count rate. For this reason, in Geiger mode operation, single photon detection is better for conventional p-i-n APDs than for SAM-APDs despite the latter showing higher linear mode gain.

3.5. Heterojunction APDs As discussed above, the use of a SAM-APD design can improve the absorption and give higher gain; however, it does not improve the Geiger mode operation. An alternate approach to addressing the absorption in GaN APDs is to adopt a heterojunction design for the APD.29 The bottom n-GaN layer begins absorbing immediately at the GaN/AlN interface. Because of the defects present at this interface the diffusion length of holes generated here is very short. This limits the hole injection from the n-layer and leads to a low quantum efficiency30 which will place an upper limit on the maximum single photon detection efficiency in Geiger-mode operation.31 The absorption can be shifted towards the i-GaN region by inserting an AlGaN layer between the AlN and the n-GaN (Figure 7: left). The n-GaN layer is retained, as it help to favor minority carrier hole injection into the multiplication region.13 This also helps to reduce the lattice mismatch at this interface by distributing the mismatch over two interfaces. Using a similar approach, the EQE of GaN/AlGaN photodiodes were improved up to 70% in another work.32 However, use of such a device structure does not eliminate all interface issues—dislocations can still propagate from the n-AlGa(In)N layer into the multiplication region.33

n-contact

Passivationp-contact

p-GaN

n-GaN

Sapphire (0001)

AlN (500nm)

i-GaN

LT- AlN buffer

n-contact

n-AlGaN

Figure 7. Left) Structure of a heterojunction p-i-n APD. Right) Spectral response for a reference p-i-n homojunction device (blue), and the heterojunction device shown schematically on the left (both devices are operated at a reverse bias of 20 V).

The disadvantage of using s heterojunction is that although the AlGaN is transparent near the band-edge of GaN it will posses its own cutoff beyond which the AlGaN will begin to absorb and degrade the EQE. In order to verity the cutoff of the AlGaN the spectra of the APD was measured under 20 V reverse bias, just before the onset of gain (Figure 7: right). The hetero junction APD showed a peak responsivity of 126 mW/A at 360 nm, corresponding to a EQE of 43%.29 In contrast a traditional homojunction p-i-n APD showed a peak responsivity of 99 mW/A at 362 nm, corresponding to a EQE of 34%.30 For homojunction GaN APDs, shorter-wavelength photons are more likely to be absorbed closer to the AlN interface, and thus less likely to diffuse into the depletion region. The AlGaN n-layer shifts the absorption towards the depletion region and thus enhances the EQE. In addition, the n-Al0.24Ga0.76N layer introduces a short wavelength cutoff due to its absorption edge at 313 nm. This, combined with the 365 nm wavelength cutoff of i-GaN, gives a bandpass spectral response between 313 nm and 365 nm, and improves the EQE of GaN APDs in that range. The heterojunction APD shows a breakdown voltage of around 83 V, corresponding to electric field strength of 2.7 MV/cm which is almost the same as the homojunction device.30 Unlike the SAM-APD approach, the heterojunction does not change the electric field in the multiplication so we expect to see the same electric field build-up and the same breakdown voltage. This means that any enhancement is likely to be due to enhanced hole diffusion by avoiding



absorption neat the recombination centers that dominate at the AlN/GaN interface of a conventional homojunction device. The heterojunction device showed a maximum linear mode gain of 3,800 under illumination with 325 nm light. 29 This gain is comparable that of the convention APD which showed a maximum linear mode gain of 5,600.30 Despite the higher quantum efficiency possible with the heterojunction design, the overall gain is not significantly higher. This was unexpected since the defects at the AlN/GaN interface was expected to limit the injection of holes into the multiplication region and spectrum suggested that the injection was improved at low bias. The hetrojuction approach does not seem to be valuable for GaN APDs.

3.6. Summary In summary the design of a high performance GaN based avalanche photodiode is requires taking into consideration the carrier multiplication dynamics, and the design of both the absorbing region and the multiplication region. Through extensive study we have evaluated both hole-initiated and electron-initiated multiplication and found that hole-initiated multiplication consistently yielded higher linear mode multiplication. This leads to the choice of back illumination for the design of GaN APDs due to limitations on the growth that makes n-on-p designs difficult to realize. Various absorption regions designs have been investigated. We found that the use of a heterojunction did not yield significant improvement over a homojunction device, but that the thickness of the bottom n-GaN layer was very important. The thickness of the active region was also a key factor as it allows for balancing the breakdown voltage and leakage current with the lengths available for carriers to undergo multiplication. The single biggest gain is linear mode device performance was realized by adopting a SAM-APD design. This resulted in almost an order of magnitude improvement in the linear mode gain, but this design was not without its limitations. The SAM-APD has a much more sluggish breakdown that makes it unsuitable for Geiger mode operation.

4. GAIN UNIFORMITY In an APD the gain ideally results from impact ionization events that take place uniformly throughout the multiplication region. However in real devices the electric filed profile exhibit a spatial variation. Small variations are expected due to spatial variations in the contact resistance, current spreading layer resistance, and the active region background carrier concentration. However, GaN devices can have defect densities as high as 109 cm−2. These defects can act as charge centers leading to a large local variation in the electric field that create hot-spots in the device where thermal run-away occurs.34,35 These hot spots are called micro-plasmas due to their distinct visible and UV luminescence. The edges of the device can also exhibit higher gain due to the surface defects that arise from device processing—this is termed edge gain. In order to evaluate the spatial uniformity of GaN p-i-n homojunction APDs a 325nm HeCd laser can be focused to a spot as small as 2 μm in diameter to selectively illuminate the device. This spot can then be rastered over the back surface of the device in order to selectively generate carriers. The photocurrent can then be measures from a series of different point to reconstruct a spatial map of the gain. The spatial uniformity of a 25 μm diameter APD was measured as a function of applied reverse bias (Figure 8). At 10 V of reverse bias the responsivity of the APD is fairly uniform over the surface of the APD. A slight depression can be seen in the center of the device, under the top metal contact. As the bias is increased to 70V, jus before the onset of breakdown, the uniformity of the APD response is seen to improve. Above breakdown, at a reverse bias of 80 V, the device become slightly less uniform—showing a slight preference for the center of the APD. However the gain is fairly well distributed over the area of the device and no signs of hot-spots, micro-plasmas, or edge induced gain are observed.



10V 20V 30V 40V

50V 60V 70V 80V

Figure 8. Spatial gain profile of a 25 mm diameter APD under reverse bias ranging from 10 V to above breakdown at 80V. The height data is normalized for image to exagerate the spatial variation. Darker coresponds to higher gain.

5. GEIGER MODE AVALANCHE OPERATION Geiger-mode operation under gated quenching has been previously demonstrated in front-illuminated GaN APDs with a single photon detection efficiency (SPDE) of 13% at a dark count rate of 400 kHz in devices with an area of 1075 μm2.36 In contrast, SiC devices have shown a low dark count rate of 28 kHz for much larger 7854 µm2 devices, but have done this with a lower SPDE of only 3.6%.37 By developing a back-illuminated Geiger mode APD38, were previously able to realize Geiger mode APDs with a SPDE of 23%.31 By optimizing the device structure we have been able to achieve a SPDE as high as 30%.39 A Xe-lamp and a monochromator setup are used to back-illuminate the p-i-n homojunction devices via a UV fiber-optic cable. The input slit of the monochromator is adjusted to vary the photon flux. The APDs are measured in Geiger mode with a pulsed gated quenching circuit, as shown in Figure 9. A large reverse DC voltage (VDC) below breakdown is applied to the APD through a 47 kΩ current limiting resistor, and a pulsed excess voltage (ΔVp, or VAC) is coupled in to the APD through a 50 nF capacitor to push the bias above breakdown. The pulse repetition rate is 10 kHz with a pulse width of 10 ns and a dead time of 100 μs. These conditions are sufficient to prevent after pulsing from triggering a spurious response.39 A Stanford Research model SR400 gated-photon counter is employed to discriminate the dark counts from photo counts. The discriminator level is set so as to maximize the SPDE. The choice of the DC voltage (VDC), the pulse voltage (VAC), and the total voltage is important to obtaining optimum results. A total voltage (VAC + VDC) above breakdown is needed for avalanche breakdown to occur. However, if VDC is only a few volts below breakdown; primary dark carriers can go through a series of impact ionization events with non self-sustaining but high-average gain before the arrival of pulse. The probability of carriers remaining in the multiplication region when the voltage pulse arrives becomes higher as VDC gets closer to breakdown. Reducing VDC results in a lower dark count probability (DCP) up to a point. By optimizing the DC voltage (VDC) and the pulse voltage (ΔVp, or VAC) to obtain the best operating bias conditions we were able to realize Geiger mode APDs with a SPDE of 30.5% with a dark count probability of 0.57.39



AV

Fiber

Gated quenching circuitGated quenching circuitApplied

bias VDC

VDC+ΔVp

10 ns 100 µs

ph/pulse ph/pulse

VOUT

Photon time distribution

CountsNcounts

NtotalApplied

bias VDC

VDC+ΔVp

10 ns 100 µs

ph/pulse ph/pulse

VOUT


Counts

Appliedbias VDC

VDC+ΔVp

10 ns 100 µs

ph/pulse ph/pulse

VOUT


CountsNcounts

Ntotal

Ntotal: Number of bias pulses Ncounts: Number of counts registered

VO

UT

VO

UT

Figure 9. Left) Geiger mode APD testing circuit. Right) diagram of the applied bias, and corresponding output for a given input of photons of approximately 3 photons per pulse.

6. CONCLUSION In summary, we have discussed the many design elements that go into developing Geiger mode avalanche photodiodes capable of single photon detection. By adopting a back-illuminated p-i-n homojunction design we were able to APDs operating in Geiger-mode capable of single photon counting with a SPDE of 30.5%.

ACKNOWLEDGEMENTS The authors acknowledge Dr. J. Zavada of ARO, Dr. Stuart Horn of U.S. Army Research Laboratory, and Drs. Scott Rodgers and John Albrecht from DARPA for their support and encouragement.

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