GaAs passivation by low-frequency plasma- enhanced chemical vapour deposition of silicon nitride A. Jaouad, V. Aimez and C ¸ . Aktik Passivation of GaAs by silicon nitride (Si x N y ) deposition using low- frequency PECVD (LF PECVD) is presented. The high amount of hydrogen implantation during this process enhances the passivation effect, demonstrating for the first time the unpinning of the Fermi level by a simple deposition of Si x N y on a deoxidised GaAs surface. The (NH 4 ) 2 S=Si x N y passivation is also simplified, and MIS capacitors are fabricated by a novel process, which consists in exposing the GaAs surface directly to sulphur solution, without the usual deoxidation etching step, followed by the deposition of LF PECVD Si x N y . Good modulation of the surface potential is observed, and the interface state density (D it ) as measured from 1 MHz C–V characteristics has a minimum of 3  10 11 cm 2 eV 1 . Introduction: The development of simple and low-cost efficient processes for GaAs passivation is highly desirable to exploit fully the performance of many optoelectronic and electronic devices based on this material, such as photodetectors, FETs, HEMT, HBT, etc. The extremely high density of surface states in GaAs results in pinning of the Fermi level at the surface close to the middle of the gap, which in turn causes a loss of carriers by the high surface recombination rate and limits both the electrical and optical properties of devices. The pinning of the Fermi level is widely attributed in the literature to As antisite defects, which are associated with the excess elemental As formed by the solid-state reaction of the native oxide with the underlying GaAs [1]. Intensive work has been carried out over the last 40 years to achieve thermodynamically and electrically stable insulator=GaAs heterostructures; various passivation techni- ques that tend to unpin the Fermi level have been developed. Among insulators, silicon nitride (Si x N y ) deposited at low temperature by PECVD, and which is widely used in silicon technology as a protective or passivation layer, seems to be attractive for GaAs passivation since it is hard, transparent, chemically inert and presents a good barrier for moisture. The effective passivation of GaAs by (NH 4 ) 2 S x =Si x N y has been reported [2, 3], and MIS capacitors have been fabricated using silicon nitride deposited by direct PECVD on GaAs after sulphur treatment of the surface. Sulphur passivation is generally realised in two steps: first, GaAs surface is deoxidised using an etch solution, then immediately exposed to (NH 4 ) 2 S x solution. The reported experiments with PECVD- deposited silicon nitride on GaAs have used a typical 13.56 MHz as radio- frequency (RF) plasma excitation. The choice of this frequency in semiconductor technology is dictated by convention and the availability of RF technology suited to this allocated frequency, rather than by optimisation of the physical processes in the discharge. In a previous work [4], we reported that silicon nitride deposited by LF PECVD on (NH 4 ) 2 S x passivated GaAs can ensure better passivation properties with respect to the same process using high-frequency (13.56 MHz) PECVD (HF PECVD). By low frequency, we imply that the excitation frequency (90 KHz) is significantly lower than the plasma frequency of about 4 MHz [4]. In this Letter we report the high GaAs passivation potential of LF PECVD that allows the fabrication of stable electrical MIS capacitors for the first time by a simple deposition of Si x N y on deoxidised GaAs substrate. We also report the first stable MIS capacitors fabricated by a simple exposition of GaAs surface to (NH 4 ) 2 S solution followed by Si x N y deposition (without any pretreatment to etch the native oxide). Experiment: An undoped n-type (100) GaAs horizontal Bridgman grown sample with electron concentration of 5–6  10 15 cm 3 was used in this study. Ohmic contact was realised by Ni=Au=Ge deposi- tion on the backside of the substrate, followed by annealing at 400  C for 60 s in N 2 atmosphere. The initial sample was cut in three pieces, in order to compare three different treatments prior to the dielectric deposition: one of the samples is deoxidised and sulphurised: the second is sulphurised without deoxidation pretreatment; and the third sample is only deoxidised and did not undergo any sulphide passiva- tion. The deoxidation is processed using NH 4 OH, HCl and HF based solutions [4] to ensure an oxygen-free surface; for sulphide passiva- tion, the samples are exposed to a (NH 4 ) 2 S commercial solution for 20 min at a temperature of 65  C. The three samples were introduced in a parallel plate PECVD reactor. A power of 35 mW=cm 2 was used, and the RF plasma frequency excitation was tuned to 90 kHz. The flow rates of silane, ammonia and nitrogen were, respectively, 540, 420 and 720 sccm, the pressure was 250 mtorr and the plate tempera- ture was maintained at 300  C. The deposited film had a thickness of 820 A ˚ and a refractive index of 2.03, as measured by ellipsometry. Aluminium dots with an area of 1.13  10 2 cm 2 were then patterned on the Si x N y surface. Fig. 1 Electrical measurements for unannealed structures 1 MHz C–V and G–V characteristics  deoxidised then sulphurised sample – – – – – deoxidised=non-sulphurised sample ———– non-deoxidised=sulphurised sample Inset: Interface trap density against energy in bandgap Results: Fig. 1 shows the typical high-frequency C–V and G–V characteristics measured at 1 MHz for the fabricated MIS capacitors. From the C–V curves, we can see that all three structures show good modulation of the surface potential; inversion, depletion and accu- mulation regions are clearly defined, indicating unpinning of the Fermi level even for the non-deoxidised=sulphurised and for the deoxidised=non-sulphurised surfaces. Fig. 1 also shows the G–V curve for the MIS structures: note the presence of one conductance peak whose position is close to the flatband voltage for each structure. A positive shift was observed in both C–V and G–V curves, which indicates the presence of high density of negative fixed charges at the Si x N y –GaAs interface. The C–V curves of the three MIS capacitors also show a comparable stretch-out along the bias axis, which is a sign of comparable D it . Indeed, as shown in inset of Fig. 1, representing D it against energy in the bandgap as estimated using the Terman method, a comparable distribution is observed for the three samples with a minimum of 7–8  10 11 cm 2 eV 1 . Fig. 2 Electrical measurements for annealed structures 1 MHz C–V and G–V characteristics  deoxidised then sulphurised sample – – – – – deoxidised=non-sulphurised sample ———– non-deoxidised=sulphurised sample Inset: Interface trap density against energy in bandgap After annealing in N 2 atmosphere at 400  C for 60 s, the 1 MHz C–V characteristic of the deoxidised=non-sulphurised sample presents the best modulation of surface potential, and the most significant reduction of the flatband voltage as shown by the C–V and G–V curves in Fig. 2, ELECTRONICS LETTERS 5th August 2004 Vol. 40 No. 16