Journal of Crystal Growth 189/190 (1998) 415—419

Optical transitions in cubic GaN grown on GaAs(1 0 0)substrates by metalorganic vapor-phase epitaxy

Jun Wu!,*, Hiroyuki Yaguchi!, Kentaro Onabe!, Ryoichi Ito!, Yasuhiro Shiraki"! Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

" Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan

Abstract

We investigated the optical transitions in cubic GaN films grown on GaAs(1 0 0) substrates by metalorganicvapor-phase epitaxy. The cubic GaN films show good optical quality. From temperature and excitation intensitydependence, the emission lines at 3.274 and 3.178 eV were assigned to the excitonic transition and the donor—acceptorpair transition, respectively. We also suggested an additional acceptor level (E

A{+212 meV) to explain the origin of the

emission lines at 3.088 and 3.056 eV, on the basis of the excitation intensity dependence. ( 1998 Elsevier Science B.V.All rights reserved.

PACS: 78.55.Cr; 81.05.Ea; 81.15.Gh

Keywords: MOVPE; Cubic GaN; Photoluminescence; Excitonic transition; DA pair transition; Free to boundtransition

1. Introduction

Cubic GaN is theoretically expected to possesssuperior electronic properties, such as higher mo-bility resulting from its reduced phonon scatteringin higher crystallographic symmetry [1]. The epi-taxially grown cubic GaN films can be cleavedalong with the substrate facet, which enables us toprepare cavity mirrors for laser diodes easily. Fur-

*Corresponding author. Fax: #81 3 3481 4509; e-mail:wujun@photonics.rcast.u-tokyo.ac.jp.

thermore, cubic GaN is expected to form a coher-ent heterostructure between GaN and other III—Vsemiconductors. Despite these possible advantages,intensive attention so far has been focused on hex-agonal (wurtzite) GaN grown on sapphire and SiC.The imbalance of research between the two phasesis due to the difficulty in obtaining good qualitycubic GaN. The inferior crystal quality and themixing of hexagonal GaN obstruct explicating theoptical properties of cubic GaN.

However, there are some recent reports on theoptical properties of cubic GaN by cathodo-luminescence (CL) and photoluminescence (PL)

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 3 1 1 - X

Fig. 1. Photoluminescence spectrum measured at 6 K for a cu-bic GaN film grown by MOVPE. The inset shows the FWHMof the excitonic transition is 70 meV at 300 K.

[2—10]. Due to the limitation of crystal quality, CLand PL results varied considerably among differentresearchers. Strite et al. [2] studied CL spectra ofcubic GaN and assigned the emissions at 3.267 and3.183 eV to free electron—bound hole and do-nor—acceptor (DA) pair recombination, respective-ly. Chichibu et al. [5] reported excitonic and DApair transitions at 3.257 and at 3.145 eV and theyalso studied the temperature dependence of PL andphotoreflectance (PR) peaks. Menniger et al. [6]studied the CL spectra from micro-size GaN singlecrystals and assigned the line at 3.272, 3.263, and3.150 eV to the free exciton, bound exciton and DApair transitions. However, they could not obtainthe resolved emission line in cubic GaN layers andthe CL spectra varied considerably among sampleswith different morphology [7]. As et al. [8] re-ported the PL measurements on cubic GaN layersgrown by MBE. They obtained two resolved peaksat 3.26 and 3.15 eV, which were identified as ex-citonic and DA pair transitions. However, theirspectrum showed a strong intensity of DA pairtransition (one order higher than that of excitontransition), indicating the possibility of high impu-rity density in their samples.

The aim of this paper is to provide well-resolvedPL spectra obtained from high-quality cubic GaNfilms grown on GaAs(1 0 0) substrates by metalor-ganic vapor-phase epitaxy (MOVPE). Temperatureand excitation intensity dependence was studied inorder to assign each emission line. The DA pairtransition is weaker than the excitonic transition atlow temperatures, indicating a good optical qualityas well as the relatively high purity of our samples.

2. Experimental procedure

Cubic GaN films were grown on semi-insulatingGaAs(1 0 0) substrates by MOVPE. Trimethylgal-lium (TMG) and 1,1-dimethylhydrazine (DMHy)were used as the precursors of Ga and N, respec-tively. The cubic GaN films were grown usinga two-step growth process. After the deposition ofa 20 nm thick buffer layer at 575°C, about 1 lmthick GaN layer was deposited at 900°C. Growthconditions in detail were reported previously [9].Photoluminescence was excited by the 325.0 nm

line of a He—Cd laser. PL signals were dispersed bya grating monochromator with a 32 cm focal lengthand detected phase sensitively using a HamamatsuR-955 photomultiplier. The excitation intensitywas controlled by a UV neutral density filter withdifferent attenuation.

3. Results and discussion

Fig. 1 shows the photoluminescence spectrummeasured at 6 K for a cubic GaN film grown byMOVPE. The spectra exhibit strong near-bandemissions. Four resolved peaks at 3.274, 3.178,3.088 and 3.056 eV can be observed from this spec-trum. There are no emission signals above 3.30 eV,which correspond to hexagonal GaN. Comparingour results with those reported in Refs. [2,5], weassign the 3.274 and the 3.178 eV emissions as theexcitonic transition and DA pair transition, respec-tively. The excitonic emission shows the strongestintensity in the spectrum and can be observed up toroom temperature. The full-width at half-maximum(FWHM) of the excitonic transition is 70 meV. ThisFWHM value is the same as the best one reportedto date by Nakadaira and Tanaka [10].

We plotted the temperature dependence of en-ergy positions for these transitions of 3.274, 3.178and 3.088 eV in Fig. 2, represented by the squares,circles and triangles, respectively. The intensity of

416 J. Wu et al. / Journal of Crystal Growth 189/190 (1998) 415–419

Fig. 2. Temperature dependence of the energy position for thepeak at 3.274, 3.178 and 3.088 eV, represented as solid squares,solid circles and triangles, respectively.

Fig. 3. Photoluminescence spectra measured at different excita-tion intensities. Note that the intensity of excitonic transitionincreases rapidly with increasing excitation intensity.

the 3.056 eV emission becomes too weak with in-creasing temperature to study its temperature de-pendence. For the excitonic transition at 3.274 eV,a peak shift to lower energy was observed. Thepeak position shift with increasing temperatures isdue to the variation of the interband transition withtemperature, which was studied in detail usingphotoreflectance in Refs. [3,5]. On the other hand,the energy position of 3.178 eV emission shifts tohigher energy, which is consistent with the behaviorof DA pair transitions. As for the 3.088 eV emis-sion, little is known about its origin at present.Because the temperature dependence of this peakdoes not follow the variation of band gap or DApair transition with temperature, we can rule outthe possibility that the 3.088 eV emission is an LOphonon replica of either excitonic transition or DApair transition.

To provide further information on the origin ofthe luminescence features, we showed the PLspectra measured at different excitation intensitiesin Fig. 3. Besides the peak shift, line intensity alsoshows variations with increasing excitation inten-sity. When using a weak excitation intensity, therelative line intensity of the DA pair transition tothe excitonic transition is strong. With increasingexcitation intensity, the line intensity of excitonictransition increases rapidly. This behavior can be

interpreted by a power law described as

IJJk,

where I is the intensity of the emission line and J isthe excitation intensity. k is a coefficient. The coef-ficient k is generally 1(k(2 for the excitonictransition and is less than one for DA pairtransition [11]. On the basis of this law, a fasterincreasing rate for the line intensity of excitonictransition is expected with increasing excitationintensity.

We showed the intensity dependence of the en-ergy position for each emission in Fig. 4. No energyshift was observed for the 3.274 eV emission withdifferent excitation intensity, which can be expectedfor an excitonic emission. With increasing intensity,the energy shift towards a higher energy was ob-served for the 3.178 eV peak, which follows thebehavior of the DA pair transitions. For DA pairtransitions, the distant pairs are responsible for thelow energy part of the DA pair band. Because therecombination rates are very low for these distant

J. Wu et al. / Journal of Crystal Growth 189/190 (1998) 415–419 417

Fig. 4. Energy position shifts with excitation intensity for eachemission line.

pairs, the recombination rates tend to saturate withincreasing excitation intensity. Thus, an energyshift upwards in energy is expected for DA pairtransitions.

For the 3.088 eV emission shown in Fig. 4 astriangles, a total energy shift of 3 meV was observedfor 3.088 eV peak when the excitation intensity waschanged by two orders of magnitude. This is prob-ably expected for a free-to-bound transition, whichinvolves the participation of free carriers. As theexcitation intensity increases, the free carrier con-centration and hence the quasi-Fermi level in-creases, resulting in a peak position shift to higherenergy [12,13]. Since GaN exhibits a shallow do-nor, we suggest that this transition is from therecombination between free electrons and holesbound to the acceptor, or free-to-bound (e A@),involving an additional acceptor level that islocated about 212 meV above the valence band.

With regard to the emission line at 3.056 eV, twofeatures can be observed: (1) Relative intensity de-creases rapidly with increasing intensity and tem-

perature. (2) An obvious energy shift was observedwith increasing intensity. These features seem tosuggest that this emission is a donor—acceptortransition (DA@), resulting from the additional ac-ceptor level (E

A{).

It is thus likely that besides the shallow acceptorlevel (E

A+90 meV) which is responsible for the

commonly observed DA pair transition, an addi-tional acceptor level (E

A{+212 meV) may be in-

volved in cubic GaN. This additional acceptor levelis responsible for the observed emission lines at3.088 and 3.056 eV. Note that in these transitions,the donor level is the same as that in DA pairtransition and only the acceptor level is different.

4. Conclusions

In summary, we have studied the photolumines-cence properties of cubic GaN films grown onGaAs(1 0 0) substrates by MOVPE. The cubicGaN films show good optical quality as well assubstantial purity. Even at a low temperature (6 K),the PL spectra are dominated by the excitonictransition of 3.274 eV. The 3.178 eV emission wasassigned to the DA pair transition by studying thetemperature and intensity dependence of the PLspectra. We also suggested an additional acceptorlevel (E

A{+212 meV) to explain the origin of the

emission lines at 3.088 and 3.056 eV. An excitonicemission at 3.216 eV with FWHM value as small as70 meV was observed at 300 K.

Acknowledgements

The authors would like to express their appreci-ation to S. Akiyama, G. Biwa, T. Tsujikawa, M.Ishikawa and M. Sato for their cooperation in theMOVPE growth, and to N. Usami and S. Ohtakefor their technical support.

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