Arrangement of nitrogen atoms in GaAsN alloys determined by scanning tunneling microscopy

Abstract

The pair distribution function of nitrogen atoms in GaAs0.983N0.017 has been determined by scanning tunneling microscopy. Nitrogen atoms in the first and third planes relative to the cleaved (1 0) surface are imaged. A modest enhancement in the number of nearest-neighbor pairs particularly with [001] orientation is found, although at larger separations the distribution of N pair separations is found to be random.Considerable interest has developed in recent years concerning GaAsN and InGaAsN alloys with low N content, typically a few %. The large predicted band gap bowing in this system of highly mismatched anions leads to the possibility of considerable band gap reduction with modest N content [1,2]. Important applications include lasers with wavelength in the 1.3-1.55 µm range, as well as solar cells with band gap around 1.0 eV [3]. Generally speaking the GaAsN and InGaAsN alloys have displayed evidence of inhomogeneities, such as broad photoluminescence (PL) line widths, variable PL decay times, and short minority carrier diffusion lengths [4][5][6][7]. Such observations are often taken as an indicator of compositional fluctuations in the materials, although direct structural characterization of such fluctuations is lacking.In this work we use cross-sectional scanning tunneling microscopy (STM) to directly probe the arrangement of N atoms in GaAs 0.983 N 0.017 alloys. Nitrogen atoms of two distinct contrast levels are imaged, which we assign to occupation in the first and third surface planes relative to the (1 0) surface. From an accurate determination of the position of about 1000 N atoms in a continuous strip of alloy material, we compute the distribution function of pair separations. The arrangement of N atoms is found to be quite consistent with that expected from random occupation, with the exception that an enhanced occurrence of nearest-neighbor N pairs is found.The GaAsN alloys studied here were grown on GaAs(001) substrates by metal organic vapor phase epitaxy (MOVPE) at temperatures between 530 and 570 C using TMGa, TBAs or AsH3, and tertiarybutylhydrazine (TBHy) under hydrogen carrier gas. Additional details of the growth and characterization of the material can be found in Ref. [8]. The particular film studied here consists of a GaAs buffer layer followed by a GaAs 0.983 N 0.017 layer, a 52 nm thick GaAs spacer layer, a GaAs 0.972 N 0.028 layer, and a 370 nm thick GaAs cap layer. The thickness of the GaAsN layers was determined by high-resolution x-ray diffraction (HRXRD) to be about 18 nm; STM measurements of their thickness gave results of 14-19 nm depending on location in the wafer. The N contents quoted above were also determined by HRXRD; STM measurements for those quantities gave similar results. The GaAs substrate, buffer layer, and cap layer were doped with Si at a con-1 1°P ublished in Appl. Phys. Lett. 78, 82 (2001).

Structural properties of GaAsN∕GaAs quantum wells studied at the atomic scale by cross-sectional scanning tunneling microscopy

“…The darkest spots labeled A are N As substitutional impurities in the surface plane ͑first atomic layer͒. 9,12 Similar features with a reduced contrast indicate substitutional N in the third and fifth planes below the surface ͑labeled B and C, respectively͒. The different intensity profiles shown in the inset of Fig.…”

“…The distribution of N atoms in a metalorganic vapor phase epitaxy ͑MOVPE͒ grown GaAsN alloy with 1.7% N was studied by X-STM a few years ago. 9,10 Three different N-related features on the ͑1−10͒ surface were found and no evidence of clustering was observed. 9,10 Nevertheless, the N distribution homogeneity could depend on the amount of N in the alloy and in the growth conditions, which differ considerably between the MOVPE and molecular beam epitaxy ͑MBE͒ techniques.…”

“…8 All these have motivated a strong effort in the last years on the structural characterization of ͑In͒GaAsN alloys, but only very few of these studies were performed using cross-sectional scanning tunneling microscopy ͑X-STM͒. [9][10][11] This technique is very useful because it allows to image the cross section of a QW or bulk layer with atomic resolution and distinguish between the different atoms in the alloy. The distribution of N atoms in a metalorganic vapor phase epitaxy ͑MOVPE͒ grown GaAsN alloy with 1.7% N was studied by X-STM a few years ago.…”

Distribution of nitrogen atoms in dilute GaAsN and InGaAsN alloys studied by scanning tunneling microscopy

“…These characteristics have been interpreted in terms of compositional fluctuations in the materials [7,8]. However, recent cross-sectional scanning tunneling microscopy (STM) results [9] have shown little evidence of clustering in GaAsN alloys.…”

“…In previous work [9] we used cross-sectional STM to image N atoms in GaAs0.983N0.017 alloys. Nitrogen atoms in the first and third planes relative to the (1 0) surface were identified.…”