Journal of Crystal Growth 189/190 (1998) 349—353

MBE growth and doping of III—V nitrides

H.M. Ng, D. Doppalapudi, D. Korakakis, R. Singh, T.D. Moustakas*Photonics Center, Boston University, 8 St. Mary+s St., Boston, MA 02215, USA

Abstract

We report on the growth and doping of GaN by molecular-beam epitaxy on the c-plane of sapphire. We find that thesteps of nitridation and low-temperature buffer have a significant effect on the structure, microstructure, defects andopto-electronic properties of the grown GaN films. The electron mobility in Si-doped GaN films was found to becontrolled both by the density of ionized impurities and the density of dislocations. This result is consistent with themodel which assumes that dislocations introduce acceptor centers. Films were doped p-type with Mg with carrierconcentration up to 6]1018 cm~3, but relatively low hole mobilities (0.3 cm2/V s). These low mobilities can be improvedby thermal annealing, a result attributed to the removal of static disorder. ( 1998 Elsevier Science B.V. All rightsreserved.

Keywords: GaN; Growth; Doping

1. Introduction

The study of epitaxial growth and doping ofIII—V nitrides by various deposition methods is oneof the most active science areas in the developmentof nitride compounds. Since nitride substrates areunavailable, films are generally grown hetero-epitaxially on foreign substrates. From the varioussubstrates investigated, the basal planes of sapphireand 6H-SiC have emerged as the technologicallyimportant ones for various optical and electronicdevices.

Early work indicates that growth of GaN on(0 0 0 1) sapphire is generally three-dimensional. To

*Corresponding author. Fax: #1 617 353 9844; e-mail:tdm@bu.edu.

alleviate this problem, Amano and co-workers [1]developed a low-temperature AlN-buffer followedby the growth of GaN at higher temperatures. Thisled to significant improvement in the morphologyand opto-electronic properties of GaN films. TheBoston University (BU) group [2] and the Nichiagroup [3] reported independently the developmentof a GaN buffer instead of AlN. Moustakas andco-workers [4] have shown that this two-stepgrowth process leads to two-dimensional growthwith a lateral growth rate 100 times faster than thevertical growth rate with very low two-dimensionalnucleation rate (&20 nuclei/lm2 h). For growth onsapphire substrates, the BU group reported also theimportance of surface nitridation (conversion ofAl

2O

3to AlN) using activated nitrogen from an

ECR source [5]. Recently, it was reported by the

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 2 9 1 - 7

Santa Barbara group [6] that such a nitridationprocess is also important during growth of GaN bythe MOCVD process and is carried out by NH

3pretreatment of the substrate. Both groups foundthat this initial nucleation layers affect the structureand microstructure of the films grown during thehigh temperature step.

The study of the heteroepitaxial growth of III—Vnitrides is further complicated by the fact that thesematerials can exist in both the wurtzite and zinc-blende polymorphs [2]. It was found that substra-tes with hexagonal symmetry stabilize the wurtzitestructures while those with cubic symmetry stabil-ize the zinc-blende structures. The two structureshave either the hexagonal close packed (hcp) stack-ing sequence along the (0 0 0 1) direction, or theface-centered cubic (fcc) stacking sequence alongthe (1 1 1) direction. The cohesive energies of thetwo structures are approximately the same andthus the formation of stacking faults and the nu-cleation of second phases during the growth ofeither of the two phases is highly probable and hasbeen observed experimentally [7—9].

Doping (both n- and p-type) of GaN has beenaccomplished by the incorporation of Si or Mgduring film growth. It was reported that Mg-dopedGaN films grown by the MOCVD process requireeither a low-energy electron beam irradiation(LEEBI) or thermal annealing in a nitrogen ambi-ent to activate the acceptors [10,11]. This post-growth treatment is needed to disassociate Mg-Hcomplexes. On the contrary, Mg-doped GaN filmsgrown by the MBE process do not require anypost-growth treatment [12].

In this paper, we discuss our recent progress inthe growth and doping of GaN films by the MBEprocess. Specifically, we address the role of thenucleation layers in the structure and properties ofthe films and the role of dislocations in the lateraltransport, as well as the mechanism of Mg incorpo-ration and its effect on static disorder.

2. Experimental procedure

The films investigated in this study were grownon c-plane sapphire by microwave plasma electroncyclotron resonance-assisted molecular-beam epi-

taxy (ECR-MBE). The growth apparatus consistsof a Varian GenII MBE unit with an ASTeX com-pact ECR source mounted in one of the effusionports. The design of this source to produce a suffi-cient amount of activated nitrogen for the growthof high-quality GaN at the rate of about 200 nm/hwas reported previously [13].

The films were grown in three steps. First, thesurface of the substrate was converted from Al

2O

3to AlN by exposing the substrate to a nitrogenECR plasma at 800°C. Evidence for such trans-formation is provided by reflection high-energyelectron diffraction (RHEED) studies. The secondstep involves the deposition of a GaN buffer(&300 A_ ) at 550°C. As discussed in the next sec-tion, the length of time of the nitridation processand the structure of the GaN buffer layer havea significant effect on the structure and propertiesof the GaN films. The final growth step takes placeat temperatures between 700 and 800°C. The filmswere doped n-type by varying the Si-cell temper-ature from 1200 to 1400°C. Similarly, the films weredoped p-type by varying the Mg-cell temperaturefrom 300 to 360°C.

3. Results and discussion

3.1. Film growth

In our earlier work, the conversion of the surfaceof sapphire from Al

2O

3to AlN was confirmed by

RHEED studies conducted using a 10 kV electrongun [5]. Recently, by using a 30 kV electron gun,we were able to observe both the AlN and theAl

2O

3RHEED patterns. From the analysis of this

data, we concluded that the AlN layer formed atthe surface by the nitridation process is rotated inthe c-plane by 30° with respect to that of sapphire[14]. In general, RHEED experiments are not ac-curate enough to be used in the determination oflattice parameters. However, in our case, we esti-mated the lattice parameter of the formed AlN bycomparing with the known lattice parameter ofAl

2O

3. The AlN formed after 5 min of nitridation is

found to have a lattice constant of 3.05 A_ and thatformed after 20 min of nitridation is 3.015 A_ . Thus,the AlN formed by this nitridation process is under

350 H.M. Ng et al. / Journal of Crystal Growth 189/190 (1998) 349–353

extreme compressive stress. This is also consistentwith results of deposited AlN layer on sapphire,which was reported to be 3.09 A_ instead of 3.11 A_[15].

The effect of nitridation on the surface morpho-logy of the substrate was also investigatedby atomic force microscopy (AFM) and the resultsare shown in Fig. 1. According to these data,the substrate becomes significantly rougher asa function of nitridation time. Our studies of thestructure, electron mobility and photoluminescenceof GaN films grown at various nitridation timesshow that optimum nitridation occurs at about10 min.

We have investigated two types of GaN buffers.In one, the buffer was grown under Ga-richconditions (Film A) and in the other under lessGa-rich conditions (Film B). We found thathigher III/V ratio led to a streakier RHEED pat-tern which is consistent with more atomicallysmooth surfaces. GaN films grown on such bufferswere examined by transmission electron micro-scopy [16]. Film B has high density of stackingfaults and a significant zinc-blende component atthe interface in the form of slabs parallel to thesubstrate. Most of these defects were limited to thefirst 100 nm of the film, and the GaN layer abovethat was of good crystalline quality. Though filmA did not have any detectable zinc-blende compo-nent, it was found to have a higher density of edgeand screw dislocations as well as inversion domainboundaries. These defects extend from the interfaceto the surface and are likely to affect lateral trans-port phenomenon. As a result, film B had higherelectron mobility than film A for the same level ofdoping. The details of these studies were publishedelsewhere [16].

3.2. Impurity doping

3.2.1. n-¹ype dopingA large number of GaN films have been doped

n-type with Si. The carrier concentration in thesefilms was varied from 1015 to above 1020 cm~3.These films were grown using various nitridationand buffer conditions as discussed previously. Theelectron mobility and net carrier concentration inthese films as determined by Hall effect measure-

Fig. 1. Effect of nitridation on the roughness of the sapphiresubstrates.

Fig. 2. Electron mobility versus carrier concentration of n-GaNfilms. The curves in the low carrier concentration regions aretheoretical curves fitted to Eqs. (1) and (2) with the indicateddislocation densities.

ments are shown in Fig. 2. The dislocation densityin the various samples of Fig. 2 was controlled bychanging the thickness and the III/V ratio duringthe growth of the buffer layer as discussed in Ref.[16]. These data do not follow the traditional be-havior in which the only scattering mechanism isthe ionized impurity scattering. The bell-shapedcurve of the data can be accounted for by consider-ing the effect of dislocations. It is well known that ifthe dislocations have an edge component, theyintroduce acceptor centers along the dislocationline which captures electrons from the conduction

H.M. Ng et al. / Journal of Crystal Growth 189/190 (1998) 349–353 351

band in an n-type semiconductor [17,18]. The dis-location lines become negatively charged anda space-charge is formed around it, which scatterselectrons crossing the dislocations, thus reducingthe mobility. This phenomenon has been studiedboth experimentally and theoretically in n-type Ge[19,20] and the electron mobility due to such scat-tering by dislocations is given by the expression[18]

k$*4-

"

30J2pe2a2(k¹)3@2

N$*4-

e3f 2j$Jm

, (1)

where a is the distance between acceptor centers(dangling bonds) along the dislocation line, f is theoccupation rate of the acceptor centers, N

$*4-is the

density of dislocations and j$

is the Debye screen-ing length,

j$"A

ek¹e2nB

1@2, (2)

where n is the net carrier concentration. Thus, theelectron mobility due to scattering by dislocationsshould increase monotonically with net carrier con-centration. As a result, the combined effect ofionized impurity and dislocation scattering wouldlead to the observed experimental results in Fig. 2.

3.2.2. p-¹ype dopingA large number of GaN films have been doped

p-type with Mg. Fig. 3 shows the resistivity for twoseries of films grown at 750 and 700°C, respectively,as a function of the temperature of the Mg cell. Atlower temperatures, the incorporated Mg is notsufficient to overcompensate the native defects. Athigher temperatures, we believe that the incorpora-tion of a high concentration of Mg causes cluster-ing of Mg atoms or incorporation in interstitial ornitrogen sites. The effect of substrate temperaturecan be accounted for by the re-evaporation of Mgbefore it incorporates into the lattice. The samplewith the lowest resistivity was found by Hall effectmeasurements to have a carrier concentration of6]1018 cm~3 and electron mobility of 0.3 cm2/V s.This value of the Hall mobility is rather low, sug-gesting that the holes move by hopping mechanism.We propose that Mg incorporation in our methodof growth induces certain degree of disorder in the

Fig. 3. Resistivity of Mg-doped p-GaN films as a function of theMg cell temperature.

Fig. 4. Effects of thermal annealing on the mobility and carrierconcentration of p-GaN films.

material which introduces band-tails in the variousenergy bands. In general, disorder can be partiallyremoved by annealing to high temperatures. Weconducted such experiments for one of our filmsand the results are shown in Fig. 4.

These data indicate that upon annealing up to850°C in a nitrogen ambient, the carrier concentra-tion of the film is reduced by roughly a factor of2 while the hole mobility was increased by morethan an order of magnitude. The origin of thisannealing effect was investigated by studying thetemperature dependence of the resistivity beforeand after annealing to 850°C. Before annealing, the

352 H.M. Ng et al. / Journal of Crystal Growth 189/190 (1998) 349–353

Fig. 5. Model for the removal of band-tails after thermal an-nealing.

activation energy was 140 meV while after anneal-ing the activation energy became 176 meV. Thischange in activation energy accounts qualitativelyfor the reduction in hole concentration and theincrease in mobility is consistent with the removalof disorder due to annealing. Based on this result,we propose that the density of states for the as-grown and the annealed films is as illustrated inFig. 5.

It is understood that the incorporation of Mg inMOCVD growth is facilitated by the formation ofMg—H complexes. In the case of ECR-MBE, webelieve that the Mg incorporation is facilitated bythe abundance of electrons arriving at the surface ofthe film from the ECR source; thus the Mg entersthe lattice in its charged state which requires muchless energy than the formation of a neutral accep-tor.

In conclusion, we have shown the importance ofthe early nucleation stages (Al

2O

3nitridation and

growth of low-temperature GaN buffer) in control-ling the structure and properties of the films. Ourn-type doping studies show the effect of disloca-tions on the lateral electron transport in goodagreement with existing theories. Our p-dopingstudies show that Mg incorporation is affected byboth the Mg cell temperature as well as the substra-te temperature. Films with high hole concentration

and relatively low mobility were produced withoutany post-growth annealing. The hole mobility wasfound to improve upon annealing, a result which isconsistent with the removal of static disorder.

Acknowledgements

This work was supported in part by ARPA.

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