In depth study of the compensation in annealed heavily carbon doped GaAs
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 pH=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 pH 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 (rC=0.77 Å) compared to that of Ga (rGa=1.26 Å) and As (rAs=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 (μI) and phonon scattering (μlat) according to Matthiessen's rule: .
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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