Metal contacts to n-type GaN

Contacts consisting of various single layer metals to n-type GaN have been formed and characterized. The current-voltage characteristics were measured for 17 different metals (Sc, Hf, Zr, Ag, Al, V, Nb, Ti, Cr, W, Mo, Cu, Co, Au, Pd, Ni, and Pt) deposited on the same epitaxial growth layer. The barrier height, ideality factor, breakdown voltage, and effective Richardson coefficients were measured from those metals which exhibited strong rectifying behavior. The barrier heights for these metal contacts were measured using current-voltage-temperature and capacitance-voltage techniques. It was found that an increase in metal work function correlated with an increase in the barrier height. The surface state density of GaN was approximated to be very similar to CdS and almost a factor of ten less than GaAs.     

Electrical characteristics and thermal stability of W, WSiN, and Nb contacts to p- and n-type GaN

A comprehensive annealing study of W, WSiN, and Nb refractory contacts to both p- and n-GaN was performed. Samples were examined by current-voltage (I-V) measurements. Both W and Nb contacts to n-GaN showed rectifying characteristics with Schottky barrier heights (qø_B) of 0.63 eV. For p-GaN, the I-V curves showed very leaky behavior. In contrast, I-V curves of WSiN/n-GaN were very leaky while those of WSiN/p-GaN were rectifying with qø_B of 0.8 eV. The degradation temperature of both W and WSiN contacts was 700°C and that of Nb contacts was 300°C.     

Ag/Ai schottky contacts on n-InP

Schottky contacts have been fabricated onn- InP using a Ag/Al/InP configuration where the Ag and Al thicknesses are 1000 and 40-50Å, respectively. Diodes fabricated on InP substrates withn ≈ 7 x 10¹⁶ cm_-3, have effective barrier heights, Ø_beff, of 0.4 eV and reverse bias leakage current densities of >4 A/cm² atV _r = - 3V. Appropriate heat treating at temperatures between 400–500° C raises barrier heights by as much as 0.25 eV, resulting in Ø_beff ≈ 0.65 eV and reverse bias leakage current densities less than 0.002 A/cm². Diode characteristics are found to vary dramatically with different surface preparations prior to metallization; results of x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiling studies indicate that native oxides which are predominantly InPO₄ produce superior contacts and that aluminum first reacts with the native oxide and then migrates through the silver to the free metal surface which results in the dramatic improvements observed upon annealing.     

The Cu/n-GaAs schottky barrier diodes prepared by anodization process

The metal-insulating semiconductor (MIS) Cu/n-GaAs diodes with thin anodic-insulating layer, which is formed by anodic oxidazation on the n-GaAs substrate in aqueous 4C₂H₆O₂+2H₂O+0.1H₃PO₄ electrolyte with pH=2.02; anodically untreated control Cu/n-GaAs diodes; and anodically treated Cu/n-GaAs diodes (several steps of anodization in the same electrolyte followed by a dip in diluted aqueous HCl solution and a subsequent rinse in deionized water) have been prepared. The anodization has increased the barrier heights as well as the ideality factors. We have obtained barrier heights of approximately 0.68 eV, 0.90 eV, and 0.92 eV for the control sample, anodically treated sample, and MIS sample, respectively, adding the contribution caused by image-force effect only. Thus, the barrier height has been increased by at least 140 meV. Furthermore, we have calculated a mean tunneling-barrier height of x=0.025 eV for the MIS Cu/n-GaAs Schottky barrier diode (SBD).     

GaN紫外探测器的Ti/Al接触的电学特性

在非故意掺杂的和掺Si的GaN薄膜上蒸镀Ti(24nm)/Al(nm)薄膜,氮气环境下400～800℃范围内进行退火。实验结果表明,在非故意掺杂的样品上,随退火温度的升高,肖特基势垒高度下降,理想因子升高,表面状况逐渐变差,600℃退火形成较低接触电阻的欧姆接触,比接触电阻率为3.03×10-4Ωcm2,而载流子浓度为5.88×1018cm-3的掺Si的样品未退火就形成欧姆接触,比接触电阻可达到4.03×10-4Ωcm2。     

Compositional dependence of thermal stability of refractory metal silicide schottky contacts to GaAs

Thermal stability of refractory metal silicide Schottky contacts, WSi_x, TaSi_x and MoSi_x withx = 0.6 andx = 2, to GaAs have been studied. It was found that the outdiffusion of Ga and As through the silicide films depends on the silicide composition, with much higher diffusion rate in films withx = 2 than in films withx = 0.6 and, therefore, contributes to the degradation of the Schottky barriers after high temperature anneal-ing.     

A transmission electron microscopy study of microstructure evolution with increasing anneal temperature in Ti/Al ohmic contacts to n-GaN

Al(200 nm)/Ti(20 nm)/n-GaN contacts have been studied using transmission electron microscopy (TEM) and the resulting microstructures correlated with the observed variation in specific contact resistance (ρ_c). A minimum ρ_c value of 7×10⁻⁷ Ωcm² was obtained after annealing at 550°C for 1 min in argon. Bulk metal and interfacial phases have been characterized, and explanations for the observed electrical behavior are proposed. A transition from TiN to AlN at the interface occurs between 650°C and 700°C.     

Barrier height determination for n-type 4H-SiC schottky contacts made using various metals

We have studied Schottky barrier contacts to n-type 4H-SiC with Cr, Mo, Ta, W, Au, and Ni. We have focused on effects of the metal work function, measurement technique and interface behavior on the Schottky barrier heights (SBHs). The contacts were prepared by metal deposition via high frequency cathodic sputtering on chemical vapor epitaxially grown epitaxial layers with low residual doping (2 × 10¹⁵ cm⁻³). Prior to deposition on Si-terminated surfaces, they were in-situ cleaned by Ar-ion sputtering for 5 and 10 min, respectively. The contacts have been characterized by means of current-voltage, capacitance-voltage (C-V), and internal photoemission (IPE) methods at room temperature. With deep level transient spectroscopy, interface deep electron traps have been detected with thermal ionization energies of 0.68, 0.77, and 1.04 eV, respectively. These traps have been attributed to structural defects formed during epitaxial growth termination. The diodes have relatively low reverse leakage current, but the ideality factor is larger than one. The SBHs have been determined from C-V and IPE measurements. It has been shown that the C-V data may contain errors resulting in a low SBH if electron deep traps are present in the interface region. In general, the SBH of various metals are influenced by the metal work function and also by the semiconductor surface preparation. The interface homogeneity is an important characteristic of the Schottky contacts, which may be improved by an optimized ion sputtering prior to metal deposition. We have considered the SBHs determined by IPE measurements as most reliable.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

低真空退火对GaN MSM紫外探测器伏安特性的影响

利用金属有机化学气相沉积生长的非故意掺杂GaN单晶制备了金属一半导体一金属交叉指型肖特基紫外探测器。用肖特基势垒的热电子发射理论研究了低真空下不同热退火条件对器件伏安特性的影响。Au-GaN肖特基势垒由退火前的0．36eV升高到400℃0．5h的0．57eV,退火延长为1h势垒反而开始下降。分析结果表明：由工艺造成的填隙Au原子引入的缺陷是器件势垒偏低的主要原因,Au填充N空位形成了施主型杂质是退火后势垒升高的主要原因。     

Electrical characteristics of high performance Au/n-GaN schottky diodes

Au/n-GaN Schottky diodes with the Au electrode deposited at low temperature (LT=77K) have been studied. In comparison, the same chip of GaN epitaxial layer was also used for room temperature Schottky diodes. The low temperature Schottky diodes exhibit excellent performance. Leakage current density as low as 2.55×10⁻¹¹ A·cm⁻² at −2.5 V was obtained in the LT Schottky diodes. The linear region in the current-voltage curve at forward bias extends more than eight orders in current magnitude. Current-voltage-temperature measurements were carried out to study the characteristics of the LT Schottky diodes. A typical barrier height of about 1.32 eV for the LT diode, which is the highest value ever reported, was obtained. The obvious enhancement in electrical performance makes the LT processing a very promising technique for GaN device application although the detailed mechanisms for the LT Au/n-GaN Schottky diodes are still under investigation.     

Study of Ti/W/Cu,Ti/Co/Cu,and Ti/Mo/Cu multilayer structures as schottky metals for GaAs diodes

Schottky structures with copper and refractory metals as diffusion barrier for GaAs Schottky diodes were evaluated. These structures have lower series resistances than the conventionally used Ti/Pt/Au structure. Based on the electrical and material characteristics, the Ti/W/Cu and Ti/Mo/Cu Schottky structures are thermally stable up to 400°C; the Ti/Co/Cu Schottky structure is thermally stable up to 300°C. Overall, the copper-metallized Schottky structures have excellent electrical characteristics and thermal stability, and can be used as the Schottky metals for GaAs devices.     

Ohmic contacts formation of silicon schottky diodes by screen printing and rapid isothermal processing

Rapid isothermal processing based on incoherent radiation as the source of optical and thermal energy is playing a major role in flexible fast-cycle time integrated circuits manufacturing. In this paper, we present the dark and illuminated current-voltage characteristics of silicon Schottky barrier diodes where the ohmic contacts are formed by screen printing and rapid isothermal annealing. These results are compared with evaporated contacts followed by furnace annealing or rapid isothermal annealing. In this paper, we have shown that the ohmic contacts formed by screen printing and rapid isothermal annealing are compatible with the contacts formed by evaporation process. The processing time of the screen printed ohmic contacts is significantly lower than the contacts formed by evaporation process.     

Electrical and structural properties of low-resistance Ti/Al/Re/Au ohmic contacts to n-type GaN

Titanium (15 nm)/aluminum (60 nm)/rhenium (20 nm)/gold (50 nm) ohmic contacts to moderately doped n-type GaN (4.07×10¹⁸ cm⁻³) have been investigated as a function of annealing temperature. It is shown that the current-voltage (I–V) characteristics of the contacts are improved upon annealing at temperatures in the range of 550–750°C. Specific contact resistance as low as 1.3 × 10⁻⁶ Ωcm² is obtained after annealing at 750°C for 1 min in a nitrogen ambient. X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the sample annealed at 750°C shifts toward the high binding side by 0.71 eV compared with that of the as-deposited one. It is also shown that the contact does not seriously suffer from thermal degradation even when annealed at 750°C for 30 min. Based on Auger electron spectroscopy (AES), glancing angle x-ray diffraction (GXRD), and XPS results, possible explanations for the annealing-induced improvement of the ohmic behavior are described and discussed.     

A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 10¹⁷, 1.5 × 10¹⁷ and 4.7 × 10¹⁷ cm⁻³ are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm². In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.     

The electrical characteristics of Al/p-InP Schottky contacts

In this study, it has been investigated the electrical characteristics of identically prepared Al/p-InP Schottky diodes. The barrier heights (BHs) and ideality factors of all devices have been calculated from the electrical characteristics. Although the diodes were all identically prepared, there was a diode-to-diode variation: the effective barrier heights ranged from 0.83 ± 0.01 to 0.87 ± 0.01 eV, and the ideality factors ranged from 1.13 ± 0.02 to 1.21 ± 0.02. The barrier height vs. ideality factor plot has been plotted for the devices. Lateral homogeneous BH was calculated as a value of 0.86 eV from the observed linear correlation between BH and ideality factor, which can be explained by laterally inhomogeneities of BHs. The values of barrier height and free carrier concentration yielded from the reverse bias capacitance-voltage (C-V) measurements ranged from 0.86 ± 0.04 to 1.00 ± 0.04 eV and from (3.47 ± 0.39) × 10¹⁷ to (4.90 ± 0.39) × 10¹⁷ cm⁻³, respectively. The mean barrier height and mean acceptor doping concentration from C-V characteristics have been calculated as 0.91 eV and 3.99 × 10¹⁷ cm⁻³, respectively.     

Estimated phase equilibria in the transition metal-Ga-N systems: Consequences for electrical contacts to GaN

High quality electrical contacts to GaN are required for the advancement of electronic and optoelectronic devices based on the III-V nitrides. In this study, the metallurgy of contacts to GaN and the implications for the design of electrical contacts are considered. First, phase diagrams are estimated for the transition metal-Ga-N systems. The diagrams are then used as an aid in predicting the reaction products of annealed metal/GaN contacts, to suggest materials that may be useful as thermally stable electrical contacts and to explore the role of the partial pressure of N₂ in the annealing environment on the reactions in metal/GaN contacts. It is believed that this information will be particularly useful to researchers during the early stages of contact development since very little experimental information is currently available on the GaN contact metallurgy.     

An electrical method to characterize thermal reactions of Pd/GaAs and Ni/GaAs contacts

Capacitance-voltage (C-V) and current-voltage (I-V) measurements were used to study the thermal reaction of Pd/GaAs contacts and Ni/GaAs contacts. The thickness of GaAs consumed by the metal/GaAs reaction during annealing was calculated from C-V analyses and I-V analyses. For annealing temperatures below 350°C, the Schottky characteristics of the diodes were good but the electrical junction moves into the GaAs after annealing. The amount of junction movement was calculated directly from our measurements. The diffusion coefficients of Pd and Ni in GaAs at 300°C were estimated both to be around 1.2 × 10¹⁴ cm₂/s.     

Morphological and electrical characterization of AI/Ni/n-lnP contacts with tapered insertion Ni-Layer

The addition of a thin Ni layer has improved the surface morphology of Al/n-InP contacts which show an enhanced Schottky barrier height (SBH) after rapid thermal annealing. In order to determine the optimum thickness of the insertion Ni layer, we have fabricated a unique sample in which the thickness of the insertion Ni layer tapered off in space. The improvement of the surface morphol-ogy as well as the SBH enhancement were realized by inserting 35 nm Ni layer annealed at rather low temperatures (around 450°C). The solid-phase reaction between Ni and InP might play an important role in the low-temperature formation of A1P which was responsible for the SBH enhancement.