In depth study of the compensation in annealed heavily carbon doped GaAs

doi:10.1016/j.mejo.2005.02.127

Microelectronics Journal

Volume 37, Issue 2, February 2006, Pages 158-166

https://doi.org/10.1016/j.mejo.2005.02.127 Get rights and content

Abstract

Heavily C-doped GaAs epitaxial layers with holes concentrations ranging from 10¹⁹ to 1.6×10²⁰ cm⁻³ have been grown by metal organic vapour phase epitaxy (MOVPE) using CCl₄ as C-growth precursor. The carbon doping characteristics of GaAs epilayers have been investigated by optimizing the V/III ratio and the growth temperature. Additional informations have been extracted from the evolution of the in situ reflectivity signal during the growth of GaAs:C. The appearance of discernible oscillations in the reflectivity response indicates the high carbon incorporation and the good surface quality in spite of the CCl₄ etching effect. The hole concentration tends to saturate at about 1.5×10²⁰ cm⁻³. The comparison between Hall effect measurements realized on sets of as grown and annealed layers, and theoretical calculations of the mobility lead to the determination of the compensation ratio of the samples.

The lattice matching conditions were systematically investigated by using high X-ray diffraction measurements from (004) and (115) planes. A comparison between the experimental mismatch and the one calculated with the Vegard's law allows the estimation of the possible origin of the compensation. Secondary ion mass spectrometry, scanning electron microscopy and atomic force microscopy have been used as complementary tools to characterize the films.

Introduction

All papers agree about the interesting properties of carbon doped GaAs. It is known that carbon has a lower diffusivity and negligible memory effects compared to other p-type dopants such as zinc (Zn), beryllium (Be) and magnesium (Mg) [1]. The possibility of reaching high doping levels permits its wide application for high speed devices such as heterostructure bipolar transistors or optical devices [2], [3], [4], [5]. The searching for new dopant source and high GaAs:C doping quality is still continuing [6], [7], [8]. We have fabricated a high quality p+n+GaAs tunnel diode [5] where the p+ layer is GaAs:C with a hole concentration of about p_H=1.2×10²⁰ cm⁻³ and the n+ layer is GaAs:Si with an electron concentration of about n=10¹⁹ cm⁻³. The peak current density was measured up to 50 A cm² and the peak to valley ratio was up to 20. These values depend on the n and p_H levels. Doping levels above 10¹⁹ cm⁻³ were required to give a tunnel effect. Yet, this requirement leads to a mismatch between GaAs:C and the substrate because of the smaller covalent radius of C atoms (r_C=0.77 Å) compared to that of Ga (r_Ga=1.26 Å) and As (r_As=1.20 Å). On the other hand, unintentional annealing during the growth of different active layers of the diode is unavoidable. In order to qualify our diode, two studies are needed. The first focuses on ageing effects on the diode characteristics (work in progress) and the second understands the origin of the compensation and relaxation mechanisms in GaAs:C. As shown by many authors [9], [10], [11], [12], the annealing deeply affects the electrical and structural properties of the layers. Especially, the layers become autocompensated. In spite of the diversity of works, the origin of this compensation and the relaxation mechanism are not definitively associated. Most hypotheses focus on the possible switching of carbon from a substitutional site to a neighbour substitutional or/and interstitial site to form positive or/and neutral centre.

In an attempt to correlate between the compensation and the structural and the electrical properties, samples with hole concentrations ranging from 10¹⁹ to 1.6×10²⁰ cm⁻³ were epitaxied under optimized conditions. The as grown and annealed layers were characterized by Hall effect, secondary ion mass spectrometry (SIMS), and high resolution X-ray diffraction (HR-XRD). Further informations about surface morphology were obtained by scanning electron microscopy (SEM) or atomic force microscopy (AFM).

Section snippets

Growth and in situ characterization

Carbon-doped GaAs layers were grown by atmospheric pressure metal organic vapour phase epitaxy (MOVPE) on epiready GaAs (100) substrates misoriented by 2° towards (110). The substrate temperature was varied from 530 to 700 °C, as measured by a thermocouple inserted into the graphite susceptor. Trimethylgallium (TMG), trimethylaluminium (TMA), arsine (AsH₃) and carbon tertrachloride (CCl₄) were used as precursors. The carrier gas was Pd-diffused H₂.

A wide range of hole concentrations were

Electrical properties

The hole concentrations and hole mobilities of C-doped GaAs layers were measured at 300 K using Hall effect with a Vander Pauw configuration. It is established that, at room temperature, the mobility (μ) is estimated by combining the effect of ionized impurity (μ_I) and phonon scattering (μ_lat) according to Matthiessen's rule: $(1 / μ) = (1 / μ_{I}) + (1 / μ_{lat})$ .

The calculation of μ as a function of the free carrier concentration for different compensation ratios (θ) is largely used by many authors [23], [24]

Conclusion

Heavily carbon doped GaAs layers were epitaxied by MOVPE using CCl₄ under optimized growth conditions. The growth process and the layers were in situ and ex situ characterized. The fitting of the variation of the hole concentration as a function of the CCl₄ partial pressure shows that the hole concentration saturates at maximum value of about 1.5×10²⁰ cm⁻³. The appearance of oscillations in the laser reflectometry signal is a proof of the high doping and high quality of elaborated films. Once we

Acknowledgements

We would like to acknowledge Mrs Chantal for SIMS measurements and Mrs Laugt for HR-XRD measurements. We gratefully acknowledge support by the DGRST.

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