*Corresponding author. Fax: #86 10 62322388; e-mail:zhangjp@red.semi.ac.cn.

Journal of Crystal Growth 189/190 (1998) 566—569

High-concentration hydrogen in unintentionally doped GaN

Jian-Ping Zhang*, Xiao-Liang Wang, Dian-Zhao Sun, Xiao-Bing Li, Mei-Ying KongInstitute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People+s Republic of China

Abstract

We use nuclear reaction analysis to study hydrogen in unintentionally doped GaN, and high-concentration hydrogen,nearly 1021 cm~3, is detected. Accordingly, a broad but intense infrared absorption zone with a peak at 2962 cm~1 isreported, which is tentatively assigned to the stretch mode of NH : Ga complex. The complex is assumed to be onecandidate answering for background electrons in unintentionally doped GaN. ( 1998 Elsevier Science B.V. All rightsreserved.

PACS: 78.30.Fs; 81.55.Hi; 71.55.Eq

Keywords: Gallium nitride; Gas source molecular beam epitaxy; Hydrogen; Autodoping

1. Introduction

GaN is the most important among III-nitrides,which due to their wide band gaps are promisingmaterials for applications in ultraviolet to red-lightemission devices and high-temperature electronics.Despite the great progress made in devices basedon GaN, such as the successful application of high-ly efficient blue light emitting diodes (LEDs) [1]and the realization of continuous-wave operationlaser [2], some fundamental problems concerningmaterial quality remain important issues. One ofthem is to clarify the role of hydrogen in GaN.Hydrogen can be involved during growth because

ammonia is used as nitrogen precursor in organometallic vapour-phase epitaxy (OMVPE) and gassource molecular beam epitaxy (GSMBE).

To the best of our knowledge, because Mg—Hcomplex is responsible for the low doping efficiencyin p-GaN [3], currently intensive interest is fo-cussed on hydrogen in p-type GaN, mainly Mg-doped samples [4,5], and till now there are fewexperimental reports on hydrogen in unintention-ally doped GaN [6]. However, from the exp-erimental point of view, due to the greatelectronegativity of nitrogen and the relatively lowgrowth temperature in GSMBE, it is reasonable tosuppose that NH

3cannot dissociate thoroughly,

therefore amounts of NH or NH2

radicals mayexist in the growth ambient, and hydrogen canincorporate into GaN epilayer by this chemicalway.

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 1 9 9 - 7

In this study, we demonstrate hydrogen of highconcentration in unintentionally doped GaNgrown by GSMBE using nuclear reaction analysis(NRA). Hydrogen is only detected in samplesof background electron concentration above1018 cm~3. As a result, a broad but intense infrared(IR) absorption band peaked at 2962 cm~1 is ob-served. The local vibrational mode (LVM) at2962 cm~1 is tentatively assigned to the stretchmode of NH : Ga complex. We think our studiescan shed light on the mechanism of autodoping inGaN.

2. Experimental procedure

The GaN epilayers utilized in this study weregrown by GSMBE on double-polished (0 0 0 1)sap-phire substrates. High-purity ammonia was used asnitrogen source and temperature of the cracker cellwas set at 700°C. First, the substrate was nitrided at800°C for 5 min, then a GaN buffer layer wasgrown at 550°C for 10 min, followed by the epi-taxial process at 750°C. The thickness of the bufferlayer and the epilayer were 20 nm and 1.0 lm, re-spectively, as established from a calibrated growthrate. For depth profiling hydrogen we preferredNRA to secondary ion mass spectroscopy (SIMS)because the former requires no standards, thoughits sensitivity is usually not as good as SIMS. Theprinciple of NRA can be found in Refs. [7,8]. OurNRA experiment was carried out in a vacuumchamber with pressure of 10~5 s. The reaction19F(1H,ac)16O was used, which has a resonancereaction cross-section when the incoming 19F beamhas an energy of 6.42 MeV. To use it for profiling,we varied the energy of the incoming 19F beam, sothat the resonance energy fell at different depths inthe sample. Once the resonance reaction occurswithin the sample, the detected hydrogen is thor-oughly from the sample, for at this moment back-ground hydrogen in the chamber cannot react withthe incoming 19F beam (off-resonance) and givesno detectable signals. Theoretically, this methodhas a detection limit of 1017 hydrogen atoms percm3, but in practice, it is convenient to detect hy-drogen with a concentration of approximately1019 cm~3. We note that in our experiment samples

with background electron concentration lowerthan 1018 cm~3 gave barely detectable signals.Fig. 1 shows the hydrogen profile of a samplewith a background electron concentration of 4]1018 cm~3. There is a peak at the surface, which isdue to surface absorbates, such as water molecules.In the bulk of the epilayer, however, the hydrogendistribution is relatively flat, with an average con-centration of 9]1020 cm~3.

To see if there are any IR active vibrationalmodes related to hydrogen demonstrated in Fig. 1,infrared absorption measurements were conductedat 300 K with a Bruker IFS 120HR Fourier trans-form spectrometer. A mid-IR glowbar light source,a KBr beam splitter, and a mercury cadmium tellu-ride (MCT) detector were used. The resolution was4 cm~1. In order to reduce the continuous absorp-tion of free carriers, before the IR absorptionmeasurements the sample used in Fig. 1 was thenimplanted with He ions with fluence of 1014 cm~2

and energy of 100 KeV, and sequentially annealedin a rapid thermal annealing (RTA) system at900°C for 60 s. Fig. 2 presents the infrared absorp-tion spectrum, where the relative absorbance isshown.

3. Results and discussion

In contrast to the theoretical prediction [9],which states that due to high formation energy andhigh diffusion barrier hydrogen solubility in n-GaNmust be very low, Fig. 1 illustrates a high H con-centration, nearly 9]1020 cm~3 in the bulk of thesample. We then speculate that the hydrogen ismainly incorporated into the epilayer by a chemicalway, such as through NH or NH

2radicals, rather

by indiffusion from the growth ambient. Fig. 2demonstrates that hydrogen in unintentionallydoped GaN causes a broad but strong IR absorp-tion zone with a peak at 2962 cm~1. Several factorsare proposed to be responsible for the broadening.First, because of a large lattice mismatch with GaN,the sapphire substrate induces large strains in theepilayer that may broaden the LVMs. Second, vari-ous high-density hydrogen-defect complexes maycontribute to the broad absorption. The defectsinclude dislocations, commonly with a density of

J.-P. Zhang et al. / Journal of Crystal Growth 189/190 (1998) 566–569 567

Fig. 1. The hydrogen depth profile of a typical sample.

Fig. 2. Infrared absorption spectrum (300 K) of the samesample used in Fig. 1, but subsequently He ion implanted andannealed.

1010 cm~2 [10], intrinsic defects and those inducedby ion implantation as well. As for the peak at2962 cm~1, from the frequency we were first led toattribute it to NH bonds perturbed by the nearest-neighbour Ga atoms. We denote the mode asNH : Ga following Darwich et al. [11].

Although there are few reports on hydrogen inunintentionally doped GaN, some experiments didindicate the existence of hydrogen. Using deep-leveltransient spectroscopy (DLTS) Hacke et al. ob-served the concentration of a deep level, labeled asE2, increased as Mg was lightly doped, and was

always larger than that of E2

in undoped sample[12]. They speculated that the development ofsome crystallographic defects with the introduction

of Cp2Mg (bis-cyclopentadienyl magnesium) might

correspond to the significant increase in the E2con-

centration. However this did not seem to be consis-tent with their annealing tests, for after annealing,the E

2concentration increased further. So it is

more reasonable to speculate that E2

is a kind ofdefect always present in GaN (perhaps disloca-tions). In unintentionally doped or n-type GaN it ismore completely passivated by hydrogen comparedto that in Mg-doped GaN, where hydrogen ener-getically favors the formation of H—Mg complexes,thus resulting in an increase in the E

2concentra-

tion measured by DLTS. That E2

concentrationincreased further after annealing was just due tooutdiffusion of hydrogen.

The frequency of the H mode indicates that theH atom of the NH : Ga complex is mainly bondedto the N atom. The most probable location of theH atom is then between the N and Ga atoms. Whenthe Ga atom is substitued by a Mg atom, theconfiguration can explain the neutralization of theacceptor. As for the electrical properties of theNH : Ga complex, it may be a singly charged do-nor, since the Ga atom in the complex has a dangl-ing bond. Hence NH : Ga complex is a possiblecandidate answering for background electrons inunintentionally doped GaN. If this is the case, itmay settle a contradiction, which stems from thefact that the lattice constants decrease as the back-ground electrons increase in the unintentionallydoped GaN [13], while according to van Vechten’stheory [14], the opposite case should be true ifnitrogen vacancy is responsible for the backgroundelectrons. On the other hand, Hall effect measure-ments really indicate that background electrons arerelated to hydrogen concentration in GaN samples.A systematic research work is being carried out onthis point.

4. Conclusions

In summary, we have measured hydrogen of highconcentration in unintentionally doped GSMBEGaN, and an intense H-related infrared absorptionpeak at 2962 cm~1 was reported. The hydrogen issupposed to be one candidate responsible for back-ground electrons in unintentionally doped GaN.

568 J.-P. Zhang et al. / Journal of Crystal Growth 189/190 (1998) 566–569

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