Ion mixing of III‐V compound semiconductor layered structuresW. Xia, S. A. Pappert, B. Zhu, A. R. Clawson, P. K. L. Yu, S. S. Lau, D. B. Poker, C. W. White, and S. A.Schwarz Citation: Journal of Applied Physics 71, 2602 (1992); doi: 10.1063/1.351079 View online: http://dx.doi.org/10.1063/1.351079 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/71/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhancement of conduction‐band effective mass in III–V semiconductor alloys induced by chemicaldisorder J. Appl. Phys. 80, 6761 (1996); 10.1063/1.363804 Optical investigations on isovalent δ layers in III‐V semiconductor compounds J. Appl. Phys. 77, 6295 (1995); 10.1063/1.359097 Temperature dependence of ion‐beam mixing in III–V semiconductors J. Appl. Phys. 77, 3543 (1995); 10.1063/1.358583 Effects of atomic clustering on the optical properties of III‐V alloys Appl. Phys. Lett. 64, 2882 (1994); 10.1063/1.111403 Structural model of III–V compound semiconductor Schottky barriers J. Vac. Sci. Technol. 21, 577 (1982); 10.1116/1.571790

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Ion mixing of Ill-V compound semiconductor layered structures W. Xia, S. A. Pappert, B. Zhu, A. R. Clawson, P. K. L. Yu, and S. S. Lau Department of Electrical and Computer Engineering, University oj’ California at San Diego, La Jolla, California 92093-0407

D. B. Poker and C. W. White Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6048

S. A. Schwarz Bellcore, Red Bank, New Jersey 07701-7040

(Received 11 September 199 1; accepted for publication 6 December 199 1)

Compositional disordering of III-V compound superlattice structures has received considerable attention recently due to its potential application for photonic devices. The conventional method to induce compositional disorder in a layered structure is to implant a moderate dose of impurity ions ( - lO”/cm’) into the structure at room temperature, followed by a high-temperature annealing step (this process is referred to as IA here). Ion irradiation at room temperature alone does not cause any significant intermixing of layers. The subsequent high-temperature annealing step tends to restrict device processing flexibility. Ion mixing (IM) is capable of enhancing compositional disordering of layers at a rate which increases exponentially with the ion irradiation temperature. As a processing technique to planarize devices, ion mixing appears to be an attractive technology. In this work, we investigate compositional disordering in the AlGaAs/GaAs and the InGaAs/InP systems using ion mixing. We found that the ion mixing behavior of these two systems shows a thermally activated regime as well as an athermal regime, similar to that observed for metal-metal and metal-semiconductor systems. Ion mixing is observed to induce compositional disordering at significantly lower temperatures than that for the IA process. We have compared the two processes in terms of five parameters: (1) irradiation temperature, (2) dose dependence, (3) dose rate dependence, (4) annealing, and (5) ion dependence (including electrical effects and mass dependence). We found that the IM process is more efficient in utilizing the defects generated by ion irradiation to cause disordering. Both the physical mechanism of ion mixing and possible device implications will be discussed.

I. INTRODUCTION

A. Compositional disordering

The intermixing of elements in structures of III-V compound quantum wells and superlattices has been inves- tigated extensively in recent years.’ The layered structure of quantum wells is generally stable thermally; there is little or no intermixing between individual layers at tem- peratures as high as 1000 “C if proper precautions are taken to prevent vaporization. In 1981, researchers at the Uni- versity of Illinois reported that the disordering of an AlAs- GaAs superlattice could be greatly enhanced by diffusing a p-type dopant, Zn, into the structure at low temperatures ( 5 600 “C) .2T3 They found that the superlattice was disor- dered (intermixed to become an alloy of AlGaAs) only in the region where Zn was diffused into the structure. In areas where Zn was not diffused into the structure, discrete layers of the superlattice remained intact. It is interesting to point out that the disordered region stayed as a single crystal and epitaxial to the surrounding superlattice re- gions. Since this discovery, impurity disordering in the temperature range of 600-900 “C has been reported in AlGaAs/GaAs,’ InGaPAs/GaAs,4 InA1GaP/InGaP,5 InGaAs/GaAs,6 InGaAs/InP,7 GaInP/AlInP,8 and other systems. Shallow dopants apparently enhance the interdif-

fusion of Al and Ga or As and P atoms, causing intermix- ing of the layered structure of superlattices. The diffusing dopants can be either n or p type and may occupy either group III or V lattice sites. Van Vechten has theorized that for the case of Zn diffusion into an AlAs/GaAs superlat- tice, the disordering was due to the interaction between interstitial Zn and antisite defect complexes in conjunction with the diffusion of As vacancies by nearest-neighbor hopping.g Tan and Giisele observed phenomenologically that the enhanced interdiffusion between Ga and Al under n-doping conditions is in accordance with the Fermi-level effect where the total Ga vacancy concentration, V,,, is raised by the addition of charged vacancies to the neutral vacancies to promote interdiffusion. lo While the detailed mechanisms of the disordering are still under investigation,“~r2 the effects of enhanced intermixing under extrinsic conditions have been firmly established. ’

Compositional disordering can be induced by several techniques: fi) impurity-induced disordering, where the impurities are introduced into the structure either by dif- fusion or by ion implantation; (ii) defect-induced disorder- ing, where a stripe pattern of SiOz is deposited on the surface of the sample (usually superlattice structures of AlGaAs/GaAs); and (iii) disordering induced by solid- phase regrowth.13 We will concentrate on impurity-in-

2602 J. Appl. Phys. 71 (S), 15 March 1992 0021-8979/92/062602-09$04.00 @ 1992 American institute of Physics 2602

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duced disordering by ion implantation here. Ion implantation of different atomic species has been

shown to promote compositional disordering.‘*‘“27 High- efficiency heterostructure lasers have been fabricated as a result of compositional disordering using MeV oxygen ion implantation.25 Introduction of impurities by ion implan- tation is favored due to the control, reproducibility, and simplicity of the process. The conventional way of disor- dering is to implant a moderate dose ( 5 lOi cm - “9 of shallow dopant ions (e.g., Si for n type and Zn for p type)26 into the sample at room temperature, followed by a high-temperature annealing step (600-850 “C!) to promote interdiffusion between layers. (This process is referred to as the IA process.) Ion irradiation at room temperature alone does not cause any sign$cant intermixing of layers, although defects must have been introduced by the irradi- ation. A high-temperature annealing step must follow to cause compositional disordering. It is generally believed that compositional disordering caused by post-implanta- tion annealings is due to a Fermi-level effect, identical to that observed in diffused samples discussed previously.26

In terms of device processing, high-temperature an- nealing steps are undesirable because of processing restric- tions and the requirement of a special annealing environ- ment such as overpressure and/or capping. To facilitate the planarization of photonic devices, a low-temperature processing technology, such as ion mixing, appears to be attractive.

B. The ion mixing phenomenon

Ion mixing is a process to induce intermixing between regions of different atomic composition under the influence of an ion beam. It has long been recognized28 as an alter- native to high dose ion implantation in metal-superconduc- tor and metal-metal systems 29-32 for ion beam modification of materials.33’34 In tribology, ion mixing is used to im- prove surface properties of wear, hardness, and corrosion.35 In microelectronics, ion mixing is used to im- prove contact formation in small areas (i.e., micron size) and interconnect surface morphology.36,37 In photonics, ion mixing is potentially useful in creating new material structures suitable for light sources, detectors, and wave- guide modulators.3842

The ion mixing (IM) process may be divided into two regimes:43 the prompt regime (or prompt mixing) and the delayed regime (or delayed mixing). In the prompt re- gime, the incident ion transfers its energy to the target atoms, producing many recoils. The primary collision, pro- ducing recoils in the forward direction, is considered to have less significant contribution to ion mixing.44 Mixing in the prompt regime is mainly due to secondary collisions which are isotropic (commonly called cascade mixing) and athermal (temperature independent). Mixing in the delayed regime is considered to be a process associated with radiation-enhanced migration, which is substrate tem- perature dependent. At low temperatures, radiation-en- hanced migration has only a limited contribution to the transport of atomic species. The prompt mixing process dominates. As the substrate temperature increases, delayed

mixing becomes more significant and finally becomes the dominant mixing process. The ion mixing process, there- fore, should generally exhibit two distinct temperature re- gimes. The temperature dependence of ion mixing effi- ciency is commonly referred to as a Q-curve.29*45 The quantity Q is usually defined as the square of the diffusion length divided by the ion dose or Q = Dt/q& where D is the effective diffusion coefficient, t is the time, and 4 is the dose. Below a certain transition temperature, T,, the amount of mixing, Q, is insensitive to the temperature (the athermal region); above T,, Q increases exponentially with the temperature (the thermally activated regime). The Q- curve characteristics of ion mixing have been observed in metal-semiconductor and semiconductor-semiconductor systems. The value of T, depends on the system,& but is typically around room temperature for metal-semiconduc- tor systems, and around 200 “C for III-V compound and IV-IV layered structures.41’4U* It is, therefore, possible to induce disordering in III-V compounds using ion mixing in the thermally activated regime at temperatures substan- tially lower than those required by conventional ion im- plantation followed by high-temperature annealing (the IA process).

In the most simplistic terms, ion mixing is just ion irradiation at elevated temperatures. What are the differ- ences between ion mixing (IM) and ion implantation fol- lowed by a high-temperature annealing (IA)? It was the primary objective of this investigation to compare these two processes in terms of five parameters: (i) irradiation temperature, (ii) dose dependence, (iii) dose rate depen- dence, (iv) annealing, and (v) ion dependence (including electrical effects and mass dependence).

II. EXPERIMENT

The III-V compound superlattice structures were grown by either molecular beam epitaxy (MBE) or by metallorganic chemical vapor deposition (MOCVD) . Most of the experiments were performed with the InGaAsAnP and the AlGaAs/GaAs systems. The AlGaAs/GaAs samples nominally consisted of 100-A Alc5Gac5As and 100-A GaAs with a total of 16 periods grown on GaAs substrates. In some cases, samples with different composition and superlattice structure were also used in this investigation. The Ine53Gac47As/InP samples usually consisted of 25-A Ine53Gac47As wells and 50-A InP barriers with a total of 50 periods grown on InP sub- strates. Ion mixing experiments were carried out with var- ious ions at temperatures ranging from 77 to -858 K ( 585 “C), and with various energies. The samples were an- alyzed with MeV ion Rutherford backscattering spectrom- etry (RBS), secondary-ion mass spectrometry (SIMS), x- ray rocking curve (XRC), cross-sectional transmission electron microscopy (XTEM), and optical measurements.

Ill. RESULTS AND DISCUSSiON

A. Irradiation temperature and dose dependence

The temperature and dose dependence of the ion mix- ing behavior of the AlGaAs/GaAs was investigated in de-

2603 J. Appl. Phys., Vol. 71, No. 6, 15 March 1992 Xia et al. 2603 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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Angie (Degree)

FIG. 1. X-ray rocking curvea (200 reflection) for an Alo,Ga,,,As (100 A)/GaAs (100 A) sample with 16 periods: (a) as-grown, (b) after ion mixing at 385 ‘C with 1 x IO” Ar+ cm-’ at 190 keV, the time for ion mixing was about 20 min, (c) after ion mixing at 585 “C, and (d) after ion implantation at room temperature, followed by annealing at 650 “C for20 min. Note that the thermal annealing temperature was 65’ higher than that of ion mixing.

tail. Figure 1 shows the x-ray rocking curves (XRC) of an A&GacsAs (100 A)/GaAs ( 100 A) sample with 16 pe- riods before and after ion mixing at 658 K (385 “C!) and 858 K (585 “C) with Arf ions to a dose of 1 x 1015 cmW2 with an energy of 190 keV. The XRC for the as-grown sample shows satellite peaks (located around hO.25”) characteristic of the superlattice [Fig. 1 (a)]. The sharp peak at 0” is the substrate peak and the peak slightly to the left ( - 0.0s”) is the zeroth-order peak of the superlattice. After ion mixing at 385 “C, the integrated intensities of the satellite peaks decrease in magnitude, indicating composi- tional disordering has occurred [Fig. 1 (b)]. After ion mix- ing at 585 “C, the satellite peaks are barely detectable, in- dicating almost complete disordering in the sample [Fig. l(c)]. A third x-ray peak located around the substrate peak appeared in the XRC after ion mixing, it is likely to be the zeroth-order peak of the ion-mixed layer appearing near the zeroth-order peak of the remaining unmixed su- perlattice layer. Also shown in Fig. 1 (d) is the XRC ob- tained on a sample implanted at room temperature fol- lowed by annealing at 650 “C for 20 min (approximately the same time period for the sample implanted at 585 “C). It is clear that even annealing at a temperature 65 “C higher than the ion mixing temperature the crystal quality of this sample is very poor after the IA process, and the extent of disordering cannot be reasonably estimated from

2604 J. Appl. Phys., Vol. 71, No. 6, 15 March 1992

- m .e 5 L : c .- -.A

2 F

.e -2 25 x

.e z

3 c -

0 c (J1 .-

v-l

T

O 50 100 150 200 250 300 Depth (nm)

FIG. 2. SIMS profiles shown on a linear scale for the four samples shown in Fig. 1. The profiles showed more mixing at 585 “C than at 385 “C. The profile from the sample implanted at room temperature followed by an- nealing at 650 “C! for 20 min has peak-to-valley ratios comparable to the virgin sample. Ion intensity of AlAs was monitored under 6-keV CJ+ bombardment at 30” off normal incidence.

XRC measurements. More quantitative analytical tech- niques such as secondary-ion mass spectrometry (SIMS) and transmission electron microscopy (TEM), are needed to characterize the mixing phenomenon. Figure 2 shows the SIMS profiles of the four samples shown in Fig. 1. The results show that ion mixing at 585 “C! causes more com- positional disordering than at 385 “C!. Ion implantation at room temperature followed by annealing at 650 “C for 20 min causes much less mixing than direct ion mixing at 585 “C. The extent of mixing or interdiffusion is evidenced by an effect on the peak-to-valley height of the Al oscilla- tions in the SIMS depth profile. Curve (c), corresponding to ion mixing at 585 “C, is unique in that it displays a significant reduction in peak-to-valley heights with the greatest reduction occurring in the middle of the film, at a depth corresponding to the projected rang? An Al diffu- sion length is estimated to be about 50 A in this case. Figure 3 shows the cross-sectional TEM micrographs of the four samples shown in Fig. 1. The TEM results confirm those shown in Figs. 1 and 2.

For well-behaved systems the kinetics of interactions, such as interdiffusion, between layers in a superlattice structure can be conveniently monitored by x-ray rocking curve measurements.48-50 The decay of the normalized in- tensity, 1, is directly related to the interdiffusion coefficient Da by

Xia et al. 2604

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FIG. 3. Cross-sectional TEM micrographs of the four samples shown in Fig. 1. The top of the figure is the surface of the sample (a) as-grown, (b) after ion mixing at 385 ‘C, (c) after ion mixing at 585 “C, and (d) after ion implantation at room temper- ature followed by annealing at 650 ‘C.

a b

-f&n d =+DL, 0

(1)

where A is the superlattice period and IO is the normalized intensity before interdiffusion occurs. It was found that low angle peaks around (000) as well as satellite peaks about (400) [or (200)] can be used for the measurements.4g150 If the ion mixing process is a diffusion-like process, the vari- able t (diffusion time) in Eq. ( 1) can be replaced by 4 (the ion dose). In this case, Eq. ( 1) can be rewritten as

(2)

where K is a constant and D is the effective diffusion coef- ficient for ion mixing. The conventional diffusion length, (Dt) 1’2, can be replaced by (04) 1’2 A plot of ln(I/Io) vs . 4 should yield a straight line with a slope proportional to D, since in ion mixing the amount of mixing is depth de- pendent. (O#> I” measured by this method is an average quantity along depth. Figure 4 shows the plot of hr(l/lo) vs 4 at various temperatures for the same AlGaAs/GaAs sample shown in Fig. 1, implanted with Si + with an en- ergy of 180 keV. It can be seen that a linear relationship is reasonably observed at different temperatures, indicating that ion mixing is a diffusion-like process. The amount of intermixing, A (usually defined as the characteristic diffu- sion length), is proportional to $“.5 (i.e., A(r4°.5). On the other hand, the IA process shows an ion dose dependence of the form A@‘.3 for doses up to about 3 x 10*5/cm2 for the AlGaAs/GaAs system. 23v24 These observations suggest that the IM process is more eficient in utilizing the ions in causing interdzflusion (compare curves c and d in Fig. 1, and the SIMS profiles in Fig. 2).

The temperature dependence of ion mixing rate (or the Q curve) for this sample is shown in Fig. 5 (a). The pa- rameter Q is proportional to KD (the slope of the straight lines shown in Fig. 4). The mixing rate indicates a ther- mally activated regime with an activation energy of -0.46 eV. The activation energy for pure thermal diffusion is - 6 eV.” The transition temperature between the thermally activated and the athermal regime is -250 “C!. Using the correlation between transition temperature and the average

cohesive energy,52 the calculated transition temperature for this system is around 300 “C. This temperature dependent behavior (i.e., the Q-curve characteristics) is consistent with that observed for metal/Si systems.44 Using the same procedure, the Q curve for the InGaAsAnP system was determined [Fig. 5(b)]. The activation energy was found to be about 0.24 eV (similar to that observed in metal/Si systems33) with a transition temperature of about 185 “C.

B. Dose-rate dependence

Figure 6 shows the x-ray rocking curves of an Alo.5Gae5As (100 A)/GaAs ( 100 A) sample with 16 pe- riods after ion mixing at 585 “C with 1 X lOI5 Ar cm -’ ions at 190 keV with different dose rates [Fig. 6(a)] and after ion implantation and annealing for different times at 585 “C! [Fig. 6(b)]. It can be seen by comparing the top and mid-

g. g.

1 1 o-' o-'

0 0 2 2 Dose Dose 71 71 0’4)6 0’4)6 -8

FIG. 4. elln (I/1e) vs dose for Al,,sG~,,As/GaAs samples implanted at various temperatures. These samples (similar to those shown in Fig. 1) were implanted with Sif ions with an energy of 180 keV.

2605 J. Appi. Phys., Vol. 71, No. 6, 15 March 1992 Xia et al. 2605

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Ea=0.46ev

lo’12 4 (a) 1000/T

Ea=0.24ev 1

~,ol) 1 2 4

lb) 1000,;

FIG. 5. Ion mixing rate vs l/T (a) for AlGaAs/GaAs samples implanted with Si + (180 keV) or Ar+ (190 keV) ions. The mixing rate is propor- tional to KD [see Q. (2)]. Within experimental uncertainties, the mixing rates were the same for both ions. The solid symbols represent the net mixing rate, i.e., subtracting the rate below T, from that above T, (b) For InGaAs/InP samples implanted with Ar + (190 keV) ions.

dle spectra in Fig. 6(a) that changing the dose rate by a factor of more than 5 (i.e., increasing the implantation time of 15 min to 80 min for a given dose of Ar of 1 x lOI cm -‘) did not affect the degree of disordering. The amount of ion mixing depends only on the ion dose and apparently not on the dose rate; this independence of mix- ing on dose rate has been reported for metal/S systems.29 This observation implies that there is an equivalence of ion dose with annealing time in a normal diffusion process. On the other hand, the IA process depends on the annealing time, as shown in Fig. 6(b) where the degree of disorder-

2606 J. Appl. Phys., Vol. 71, No. 6, 15 March 1992

Angle (Degree) (4

-0.4 -0.2 0.0 0.2 0.4 Angle (Degree)

(b)

FIG. 6. X-ray rocking curves (200 reflection) for AlO,,G%h,As/GaAs with 16 periods: (a) IM process, (b) IA process. In Fig. 6(a) the bottom spectrum is for the as-grown sample, the middle spectrum is for the sample ion mixed at 585 “C with 1 X 10” Ar + cm -’ at 190 keV, the implantation time is 15 min, and the top spectrum is for the sample with an implantation time of 80 min. In Fig. 6(b) the bottom spectrum is for the as-grown sample, the middle spectrum is for the sample implanted at room temperature with 1 x lOL5 Ar+ cm * ions at 190 keV followed by annealing in forming gas at 585 “C for 15 min, and the top spectrum is for the sample implanted with the same conditions but annealed for 80 min at 585 “C.

ing was significantly larger for samples annealed for 80 min at 585 “C than for those annealed for only 15 min.

C. Post-implant annealing

Figure 7 shows the x-ray rocking curves of an Alo.,GacSAs (100 A)/GaAs (100 A) sample with 16 pe- riods before and after ion implantation followed by a prox- imity anneal at 850 “C for 2 h in an ambient of flowing forming gas. The as-grown sample shows the characteristic satellite peaks of the superlattice structure (spectrum a). Annealing this sample at 850 “C for 2 h does not cause any change in the XRC, suggesting annealing under these con- ditions does not cause disordering. A portion of this sam- ple was then implanted with Ar + ions at 77 K to a dose of 3 X 1014 cm -2 with an energy of 190 keV, followed by a proximity anneal at 850 “C for 2 h. The XRC (spectrum b) obtained on this sample indicates the absence of any satel- lite peaks, suggesting significant or complete compositional disordering due to this IA processing step. In addition, we found that samples implanted at temperatures ranging from 77 to 473 K (200 “C) followed by annealing at 850 “C showed very similar XRCs, indicating a similar degree of disordering in these samples. However, increasing the im- plantation temperature to above 573 K (300 “C) tends to reduce the degree of disordering after annealing. This can be seen from the XRC in Fig. 7 (spectrum c) for a sample implanted at 425 “C followed by annealing at 850 “C. The characteristic satellite peaks are still present although with

Xia ei a/. 2606

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0 -0.4 -0.2 0.0

Angle (Degre?? 0

FICI. 7. X-ray rocking CUIWS (200 reflection) for Al,,ZGa,,SAs/GaAs sample with 16 periods: (a) as-grown, (b) after ion implantation with Ar + ions at 77 K to a dose of 3 X lOI cm - ’ with an energy of 190 keV and annealed at 850 “C for 2 h, and (c) after implantation at 425 “C and annealed at 850 “C.

reduced intensities, suggesting incomplete disordering in the sample. It should be pointed out that the critical tem- perature, To of the Q curve for this system is between 200 and 300 “C [see Fig. 5 (a)]. It appears then that implanta- tion at temperatures below T, followed by high-tempera- ture annealings leads to a similar degree of disordering in the sample, whereas implantation at temperatures above T, (i.e., in the thermally activated regime) followed by a post annealing at higher temperatures reduces the disordering efIiciency. These results suggest that defects generated by implantation at temperatures below T, are frozen in and become active in causing disordering during the post-an- nealing process. Defects generated by implantation at tem- peratures above T, are active in causing disordering during the implantation period and become exhausted in numbers after implantation. Fewer defects are, therefore, available for causing further disordering during the post-anneal. Since simple defects such as interstitials are mobile even at low temperatures, the frozen-in defects that become active during post-implantation annealing are thought to be va- cancies.

D. Electrically active ions

For the IA process, it is often observed that electrically active ions (i.e., shallow dopants), such as Si in AlGaAs/ GaAs, enhance the rate and the range of compositional

2607 J. Appl. Phys., Vol. 71, No. 6, 15 March 1992

disordering in the super-lattice structure.“‘26’27 These re- sults are consistent with the concept of the Fermi-level effect discussed previously.” For the IM process, compo- sitional disordering occurs primarily during ion irradiation at elevated temperatures. The sample is not under equilib- rium, and the Fermi level has no significant meaning. It follows then that the disordering mechanism based on the Fermi level effect for the IA process may not apply to the IM process. X-ray measurements were used to monitor the ion mixing rate of an AlesGae5As/GaAs sample irradiated by either Si + ions ( 180 keV) or Ar + ions ( 190 keV) at different temperatures to a dose of 5 X 1Ol4 cm - 2. The slight difference in energy ( 180 keV vs 190 keV) of the incoming ions is to compensate for the projected range due to the atomic mass difference between Si and Ar. It can be seen from Fig. 5(a) that within the scattering of experi- mental data, the ion mixing rate is the same for Si as well as for Ar, over the temperature range from -585 “C to room temperature. These results indicate that electrically active ions, such as Si in AlGaAs/GaAs, do not enhance disordering as it is observed for the IA process. Similar results have also been observed for the Si/Ge system.“* The disordering mechanism under ion mixing conditions is es- sentially due to the thermal migration of defects (likely to be vacancies) whose population is apparently not affected by the presence of electrically active ions, other than that due to collisional processes during ion irradiation at ele- vated temperatures.

E. Insulating ions

From the experimental results shown in Fig. 5(a), it is clear that electrically active ions are not required to induce disordering by ion mixing. This fact presents one of the major advantages of ion mixing since the electrical conduc- tivity of the disordered region can remain unaffected. Mix- ing ions such as 4oAr + are also readily available compared to 28Si+ ions. On the other hand, when a change of the electrical property is called for, properly chosen ions can be used to cause simultaneous compositional disordering and modification of electrical properties. In photonic de- vice fabrication, it is often necessary to isolate neighboring devices electrically to reduce crosstalk. Oxygen has been reported to form donor states in InP and InGaAs.53 For this reason, oxygen ions were chosen in this study to mix a p-type InGaAs/InP superlattice structure to achieve simul- taneous compositional disordering and the formation of high resistivity material.

Figure 8 shows the x-ray rocking curves for an InGaAs (20 rf)/InP (80 A) sample with 50 periods before and after ion mixing at 5 15 “C with 1 X 1013 cm - ’ oxygen ions at 180 keV. It can be seen that all satellite peaks disap- peared after mixing, suggesting complete disordering of the structure. It should be noted that the ion dose needed to disorder the InGaAs/InP system is very small compared to that for the AlGaAs/GaAs system. Generally speaking, about 100 times higher dose is required to disorder the AlGaAs/GaAs system. Figure 9(a) shows the photolumi- nescence (PL) data at 10 K from the as-grown sample and Fig. 9(b) shows the PL data for the ion-mixed sample at

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5. Y 5 ,x E Lii 9 x .G6

ii _c

g -I

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Angie (Degree)

FIG. 8. X-ray rocking curves for InGaAs (20 &/InP (80 A) sample with 50 periods before (a) and after (b) ion mixing at 515 % with IX lOI “0 + cm- ’ ions at 180 keV.

515 “C! with oxygen. The PL spectrum of the as-grown superlattice displays two well-resolved peaks which are at- tributed to transitions originating from quantum well re- gions varying by one monolayer. An effective band gap

1100 1200

Wavelength (nm)

FIG. 9. Low-temperature (-10 K) photoluminescence (PL) spectra for a 50-period 20-A InGaAs/SO-A InP SL structure before (a) and after (b) ion mixing at 515°C with 1X1013 O+ cm-‘ions at 180 keV.

-0.4 -0.2 0.0 0.2 0.4 Angle (Degree)

FIG. 10. X-ray rocking curves for an AlGaAs/GaAs sample with 16 periods. The bottom spectrum is for the as-grown sample. Tme middle spectrum is for the sample ion mixed with 585 “C with 1 X 10” Ar cm - ’ at 190 keV. The top spectrum is for the sample ion mixed at 585 “C! with 1 X lOI Ne + cm - ’ at 120 keV. The energies of the ions were adjusted for approximately the same range.

blue-shit of 106 meV corresponding to the mixed InGaAsP alloy is obtained from ion mixing. Linewidth broadening and integrated PL intensity reduction does oc- cur due to the damage induced by the implantation pro- cess. However, due to the low implantation dose and ele- vated implantation temperature, a PL signal is readily obtained which is not the case for room-temperature im- plants at moderate dose.

F. Ion mass dependence

The comparative study of impurity induced disorder- ing has showns4 that the Si-implanted sample has higher mixing efficiency than that of the Ge-implanted sample, although Ge has a higher atomic mass than that of Si. The mixing efficiency in metal/% systems has been found to depend on the mass of the mixing ions.? A similar effect was also observed in ion mixing of semiconductor layered structures. Figure 10 shows the x-ray rocking curves ob- tained from an AlGaAs/GaAs superlattice mixed with Ne ions (Z = 10) and with Ar ions (Z = 18). It can be seen that the degree of disordering was much more significant for the Ar ion-mixed sample than for the Ne ion-mixed sample. Ion mixing is more efficient with heavier ions. It is sometimes necessary to use ions with a light mass due to range consideration. The mixing efficiency for light ions can be improved by increasing the mixing temperature. For a given superlattice system, two degrees of freedom

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(i.e., temperature and ions) are usually available to achieve the optimal disordering scheme.

G. Device applications

As discussed earlier, high-temperature processing is undesirable for device fabrication. To demonstrate ion mixing as a potentially viable device fabrication technique, we have made planar buried rib waveguides by ion mixing.@ The waveguides, operating at 1.5~pm wave- length, were fabricated using I~I~~~G~.~~As (20 A)/InP (80 A) superlattice (50 periods) structures grown by MOCVD. A metallization scheme of W ( 1500 A)/Hf ( 100 &/Au (3200 A> was used as self-aligned ion masks (5 ,um in width) for 180-keV 0 + ions. Ion mixing was per- formed at 515 “C at a dose of 1 x 1013 cm -‘. The InGaAs/ InP superlattice was found to be compositionally disor- dered by x-ray rocking curves, optical absorbance measurements, and PL measurements. Waveguide effects are routinely observed at 1.5~pm wavelength with propa- gation losses in the 1.5-dB/500+m range, which corre- sponds to an absorption rate of about 7/cm. This result is reasonable considering the fact that the absorption edge is fairly close to the operating wavelength. More work is now in progress to improve the waveguide performance and to fabricate other photonic devices such as modulators and lasers using ion mixing.

IV. SUMMARY

The ion mixing behavior of semiconductor superlattice structures has been investigated. We found that: ( 1) the ion mixing behavior of these systems shows a thermally activated regime as well as an athermal regime, similar to that observed for metal-semiconductor systems, (2) ion mixing in the thermally activated regime can lead to effi- cient compositional disordering without resorting to high- temperature post-annealings, (3) the IM has a dose depen- dence of #“.5, whereas the IA process has a dose dependence of #“.3. IM is, therefore, a more efficient pro- cess in utilizing the ions, and (4) electrically active ions do not lead to enhancement of ion mixing rates, contrary to that observed for the IA process. More readily available ions, such as Ar ions, can be used for ion mixing.

Most of the results reported here were obtained on the AlGaAs/GaAs system. However, the general conclusions also apply to the InGaAs/InP system and perhaps even to the Ge/Si system. Ion mixing appears to be potentially useful for optoelectronics applications, especially in pla- narization of devices and self-aligned technology. In order for the opto-electronic community to accept ion mixing as a processing technique, it is important to eliminate or to reduce defects generated by ion irradiation to an acceptable level. This is now the challenge and much more work is needed to address this issue.

ACKNOWLEDGMENTS

The financial support of the Department of Energy (grant No. FG03-84ER45156, Jerry Smith) is greatly ap- preciated. The work at Oak Ridge was supported by the

Division of Materials Sciences, U.S. Department of Energy under contract No. AC05-84-OR21400 with Martin Mari- etta Energy Systems, Inc. P.K.L.Y. also acknowledges the support of DARPA.

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