Flow modulation epitaxy of indium gallium nitride

Journal of Electronic Materials, Oct 1997 by Keller, S, Mishra, U K, DenBaars, S P

InGaN layers were grown on GaN films by flow modulation epitaxy (FME) using the precursors trimethylgallium, trimethylindium, and ammonia. The indium composition of the FME grown layers was generally lower than of films grown under the same conditions in the continuous growth mode, but which had been of poor optical quality. The indium incorporation efficiency increased with decreasing ammonia flush time, increasing ammonia flow during group-III injection, and increasing group-III precursor injection time. Films grown under optimized conditions showed intense band edge related luminescence at room temperature up to a wavelength of 465 nm. Atomic force microscopy investigations revealed a strong dependence of the surface morphology of the InGaN films on the growth mode.

Key words: Atomic force microscopy, flow modulation epitaxy, indium gallium nitride (InGaN), metalorganic chemical vapor deposition (MOCVD), photoluminescence

INTRODUCTION

Flow modulation epitaxy (FME) and, in particular, atomic layer epitaxy (ALE) have shown several advantages over conventional epitaxial methods, especially due to their potential of performing growth at significantly lower temperatures. Within the groupIII nitrides, low deposition temperatures are required especially for the growth of indium containing alloys. The indium incorporation efficiency strongly increases by reducing the growth temperature from 850 to 5000C..I The difficulties in the epitaxy of nitrides at temperatures below 1000C arise mainly from the low decomposition efficiency of the commonly used nitrogen precursor ammonia at lower temperatures. The pulsed and alternated precursor injection applied in ALE or FME, however, takes advantage of surface catalytic effects, causing group-V precursors like AsH^sub 3^, PH^sub 3^, and NH^sub 3^ to decompose more easily on metal atom terminated surfaces compared to mixed surfaces.2, 3 Thus, high quality GaN has been obtained by ALE at temperatures as low as 550C in a specially designed ALE reactor.4

In this paper, we studied the influence of the pulse sequence and the growth temperature on the properties of InGaN grown in a FME growth mode using a conventional metalorganic chemical vapor deposition (MOCVD) reactor. The optical and structural properties of the FME films are discussed in comparison to those of InGaN layers grown by conventional MOCVD.

EXPERIMENT

The GaN/(In,Ga)N heterojunctions were grown on c-plane sapphire in a horizontal, atmospheric pressure MOCVD reactor (Thomas Swan, Ltd.) using the precursors trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia. The (In,Ga) N films of 500-800A thickness were deposited on 1.8 lm thick GaN films 5 at temperatures between 730 and 820degC. The TMGa flow was 5 lmol/min. The TMIn flow and the ammonia flow were varied between 4.5 and 21 mol/min and 0.025 and 0.36 mol/min, respectively. The layers were doped with Si2H6 resulting in a Si concentration of 2 x 10^sup 18^ cm^sup -3^ , determined by highmass resolution secondary ion mass spectroscopy (SIMS). The total gas flow was 10.31/min. All InGaN layers were grown using nitrogen as the main carrier gas and as carrier gas for the TMIn bubbler. In the FME growth mode, two different switching schemes were investigated. In case (A), the ammonia flow was held at 0.35 mol/min during the ammonia flush time but was reduced to 0.025 mol/min prior to group-III injection as shown in Fig. 1. In case (B), the ammonia flow was kept constant at 0.35 mol/min throughout the entire growth cycle.

The layers were characterized by double crystal xray diffraction using the (002) reflection of GaN and InGaN. The In composition of the films was determined from the peak separation of the GaN and InGaN related peaks applying Vegard's law. Room temperature photoluminescence (PL) measurements were performed using the 325 nm line of an He-Cd laser operating at a pump power of 1 mW. The surface morphology of the films was characterized by atomic force microscopy (AFM) in tapping mode. The rootmean-square (RMS) roughness of the surfaces was calculated using the AFM software.

RESULTS

Figure 2 shows the effect of the ammonia flow during group-III exposure, f^sub NH3^ (III), on the optical properties of InGaN films (scheme A vs scheme B). The group-III injection time, t^sub III^, was 5 s, the ammonia flush time tv, 4 s, the growth temperature 750degC. The indium mole fraction of x^sub In^ = 0.12 obtained at a low ammonia flow during group-III injection (scheme A) was considerably lower than the indium mole fraction x^sub In^ = 0.29 of the film grown with high ammonia flow (scheme B). In addition, samples grown with a low ammonia flow during group-III injection were of poor optical quality and showed much stronger deep level related luminescence than those grown with high ammonia flow. Due to the substantially lower In incorporation into layers grown after scheme A, all further experiments had been performed applying scheme B.

The dependence of the alloy composition on the group-III injection time (Te*= 750oC, tV= 8 s) is plotted in Fig. 3. An increase of the group-III precursor injection time from 3 to 10 s caused the In mole fraction of the films to increase from 26.5 to 29.4%, respectively. The optical quality of the films was best for a group-III injection time of till = 5 s. Films grown with a group-III precursor injection time of 10 s showed only weak band edge related luminescence. The intensity of the band edge related luminescence had been comparable for films grown with group-III precursor injection times of 3 and 5 s, but at til, = 3 s considerably enhanced deep level related luminescence was observed. This behavior is presently not well understood and needs further investigation.