ÖZET Geleceğin yarıiletkeni olarak adlandırılan galyum arsenür(GaAs)'ün üzerine, silisyum üzerinde büyütülen silisyum dioksit (Si02) kalitesinde bir yalıtkan tabakanın oluşturulamaması, bu konudaki araştırmaların devam etmesine neden olmaktadır. GaAs üzerinde kaliteli bir yalıtkan tabakanın büyütülmesi, entegre devrelerin tabanını oluşturan MOS ( Metal-Oksit- Yaniletken) yapıların elektronik sanayindeki çok yüksek taşıyıcı hız ile optoelektronik uygulamalarla büyük bir atılım yapabileceği göz önünde tutularak başlatılan bu çalışmada, p-GaAs üzerine anodikleme yöntemiyle altı ayrı elektrolit kullanılarak büyütülen yalıtkan tabakaların (oksit,sülfür ve florür) GaAs-MOS yapısındaki yalıtkan - yarıiletken arayüzeyin elektriksel özelliklerine etkisi incelenmiş olup: 1. p-GaAs'ün üzerine ilk defa anodikleme prosesinde susuz amonyum sülfür ve florür elektrolitleri kullanılarak kaim sülfür ve florür tabakası büyütüldüğü; 2. Büyütülen yalıtkan tabakalarından en düşük dielektrik sabitine 293K'de florür tabakasının, 77K'de ise sülfür tabakasının sahip olduğu; 3. Çalışılan tüm p-GaAs MOS yapılardan düşük akım yoğunluğu (0.1 A/cm ) ile amonyum sülfürlü susuz elekti devre elemanı imalinde kullanılabileceği; '9 mA/cm ) ile amonyum sülfürlü susuz elektrolit kullanılarak oluşturulan yapının 4. Yukarda sözü edilen yapının 77K'deki yüksek frekans C-V eğrisinde histerezis olayının olmayışı ve diğer numunelerin 293K'deki histerezis büyüklüklerine göre de, daha düşük histerezise sahip olması, bu yapının uzun senelerdir sürdürülen ve başarısız kalan GaAs MOS teknolojisinin istediği arayüzey durum yoğunluğunu (NSSFB * 10 cm` ) sağladığı, belirlenmiştir. INVESTIGATION OF INTERFACE PROPERTIES in p-GALLIUM ARSENIDE (GaAs) / ANODIC INSULATOR FELMS SUMMARY There has not been any success in research, which began in 1966, for the development of GaAs-MOS technology which the electronics industry expects to achieve. As GaAs starts to decompose at 450° C, it is necessary that dielectric film growth be conducted at lower temperatures. Although anodic oxidation process has been considered, no real success has been obtained. Because interface states density, Nss which is the reason for the electrical unstability of GaAs-MOS structure, could not be lowered. Even though in last 10 years, another non-native insulator layer formation on GaAs instead of oxide film has been tested (GaAs-MIS structures), no such improvement has been found. The insulator layer (Si02, silicone dioxide) of technological value in Si- MOS structures is of good quality. It is essential that the insulator layer to be chemically and electronically stable. On the other hand the interface state density and charges inside the insulator should be low. This study is aimed at lowering the interface state density which is the most significant parameter in MOS technology. For this purpose the growth of sulfide and fluoride layers on p-GaAs crystals have been achieved. The determination of electrical properties ' of MOS structures (p-Ga As/Anodic Sulfide(AS)/Aluminium(Al)orp-GaAs/AnodicFluoride(AF)/Aluminium(Al)), which was manufactured by growing Al metal on insulator layer and the obtained structural properties of p-GaAs/Anodic Oxide(AO)/Aluminium(Al) were compared with the ones existed in literature. In obtaining each of the three different p-GaAs MOS structures it goes through significant stages. The first one is an ohmic contact and research on this topic still continues. Also in this study there has been some problems in making of p-GaAs/ohmic contact. Long periods of time and various VIexperiences were needed to overcome these problems. The following most important points have been considered in overcoming the problems encountered in making Zn-Au ohmic contact of good quality: 1. p-GaAs crystal surface, on which ohmic contact will be grown, should not be oxidized. 2. When vacuum evaporation method is used for ohmic contact formation, consideration should be that Zn metal is easily oxidized and could not have good adhesion properties for the semiconductor surface, and also melting points of zinc and gold are quite different. 3. Flash annealing should be made to provid the conversion from Schottky contact to ohmic contact. In this process diffusion rate of zinc into the semiconductor is high. Also arsenic can easily evaporate from the GaAs. The remaining gallium forms an alloy with gold. 4. In liquid phase alloying process, the interface between the ohmic contact and GaAs is not clear. Also the obtained ohmic contact resistance could not be reproducible, but it can only be achieved in the order of magnitude. In the light of the above mentioned cases surface polishing, surface etching, metal growing, flash annealing and current-voltage measurement techniques are applied for the manufacture of p-GaAs/ohmic contact. The ohmic contact resistance reached is found to be almost equal to the specific resistance of the semiconductor. The determined specific resistance (2- 2.2x10' Qcm ) is almost equal to the one given in literature. The second step in obtaining the p-GaAs MOS structures is an insulator growth. The encountered difficulties at this stage are given as follows: 1. The crystal surface to be anodized should not be oxidized before the anodization. 2. The obtained insulator layer should indicate homogeneous distribution and be reproducible. 3. After the anodic process the electrical properties of insulator layer should not vary with time. vn4. Minimum current density, in anodization process, could vary according to electrolytes used. 5. The linear regions with different slopes for each anodization V-t curve indicates the change in chemical composition of the layer. 6. The anodization process is a function of the current density, electrolyte type, pH and mixing conditions. 7. The insulator layer obtained by anodization is porous and may contain some electrolyte. Annealing is required to remove this electrolyte. The annealing process also contributes to electrical stability in MOS structures. 8. In order to improve the coating quality by anodization the surface removal should be repeated several times. 9. The difference between the Ga and As ions affects the anodization movement between these ions. This effect should be minimized to obtain a good insulating layer. The anodization process, in the light of the above conditions, have been carried out with oxide, sulfide and fluoride coatings. In anodization process six different electrolyte have been used. The properties of possible compounds have been investigated. The investigation reveals that the anodic oxidation of GaAs leads to Ga203and As203; anodic sulfurization with leads to arsenic sulfide (As2S3) and gallium sulfide (Ga2S3); and anodic fluoridization is presumed to result in gallium fluoride (GaF3) insulating layers. As arsenic oxide is soluble in water, some deficiency of As203 on the oxide layer surface is expected. This supposition has been proven by advanced analytical techniques. The As203 produced during anodic oxidation is expected to undergo a reduction in solubility in electrolytes. This is only possible if electrolytes are saturated with As203 prior to the process. In several studies the tartaric acid + propylene glycol (AGW) system saturated with As203 the produced oxide layer maintained at every level, including the interfaces, a ratio of As203:Ga203=l:l. However, the electrical properties of such GaAs MOS structures have not been found in any literature. Also, there is no information on the saturation of AGW electrolyte, citric acid+ propylene glycol with respect to the effect on oxidation and GaAs MOS structures preparation. Therefore, in this study, AGW (tartaric acid + propylene glycol and citric acid + propylene glycol) systems which give the best results in anodic oxidation together with the saturated solutions of these vintwo electrolytes with arsenic oxide have been used. The effect of saturating with As203 in expanded oxides on the quality of GaAs MOS structures has been investigated. The anodization process at 0.1; 0.3; 0.5 and 0.7 mA/cm current densities using the electrolytes of nonaqueous ammonium sulfide in anodic sulfurization and nonaqueous ammonium fluoride in fluoridization has been carried out. Gallium sulfide grown in anodic sulfidization can be decomposed due to the presence of air and the moisture in air. The stable crystal structure(y) of this compound at normal temperature is zincblend. Thus in sulfidization it is possible to pass through a good quality of GaAs MOS structure by minimizing the dangling bonds at the interface of insulator and GaAs surfaces. During this transitions, the insulating layer should be. protected from atmospheric conditions. At some current densities of anodic fluoridization, as in anodic oxidation, a change in the slope of cell voltage vs. time (V-t) plots has been identified. This may be attributed to the change in chemical composition in the coating layer. This behavior has not been observed in the anodic sulfidization which indicates this homogeneous structure. As in all anodization processes, the grown layer in the anodic oxidation has been identified to have an amorphous structure by X-Rays diffractometer. Removal of the electrolyte present in the amorphous structure by annealing has not been tested. Although there are a number of experimental procedures on this subject, thermal treatment has been purposely avoided because one of the objective of this study was to obtain a high quality insulator layer without heat treatment. The third important phase in the manufacturing of p-GaAs MOS structures is the metal electrode preparation. As metal, relatively pure aluminum has been used in this study. The comparison of electrical properties of p-GaAs MOS structures has been evaluated with the help of- quasi-static I-V and high frequency C-V characteristic measurements. The results are summarized below: 1. The p-GaAs/AO/Al structures produced by anodic oxidation were found to have the same properties as in similar studies reported in literature. 2. The two electrolytes saturated with arsenic oxide used in anodic oxidation has not affected the oxide quality produced in tartaric acid solutions. But some decrease in the dielectric constant of the oxide produced in citric acid solutions were established. This was ascribed to the inability of IXarsen oxide to pass into the electrolyte. This provided an insulating layer with less porosity. 3. The quasi-static I-V characteristic measurements for p-GaAs MOS (p-GaAs/AO/Al, p-GaAs/AS/Al and p-GaAs/AF/Al) structures manufactured by coating anodic oxide, sulfide and fluoride, an expected behavior for each structure, have been observed. The characteristic behavior was observed with anodic sulfidized p-GaAs/AS/Al structure sample at 0.1 mA/cm current density. Compared to the other samples, it was found to be close to the ideal low frequency MOS structure's curve. 4. The anodization quality made in this study, as in all MOS structures,is determined by the interface state density (Nss). Nss should be low for the good quality manufacturing of MOS structure. For obtaining low Nss Values interface state charge density (Qss), insulator's capacity in the accumulation region (C ) and the deviation (A V) between the high frequency C-V and ideal MOS C-V curves should be lowered. In comparing the prepared MOS qualities, the flat band state values of Ngs, Qsg and AV ( NSSFB' QSSFB' AVFB ) have been USed> 5. The CQX and AV values calculated from p-GaAs MOS structures quasi-static I-V characteristic measurements have been compared. In the flat band states the smallest AVpg has been identified in the anodic sulfidized p- GaAs MOS ( p-GaAs/AS/Al ) structure produced at 0.1 mA/cm current density. The CQX values with insulating capacities in the accumulation region vary in the range of 10 and 10` F/cm for all three different growing MOS structures. An increase in the current density of anodization process results in a decrease in Cox values. The smallest value in Cox at 0.1 mA/cm current density has been observed in anodic oxide p-GaAs/AO/Al structure at 293K. In contrast to this, the smallest value of AVpg has been measured in anodic sulfidization. The lowest NSSFB value has been identified in anodic sulfidization. 6. The reason for left deviation (AV) of high frequency C-V curve with respect to the ideal MOS curve at all p-GaAs MOS structures is the positive charges within the insulating layer. The Ga, As ionic bonds produced in the semiconductor surface during anodization are not adequately filled by oxygen, sulfide or fluoride anions. These empty dangling bonds are open to charge transfer. This leads to instability in the electrical properties of p-GaAs MOS structures. In anodic sulfidization the formation of Ga2S3 in GaAs crystal structure reduces the number of unfilled bonds. Therefore, the sulfidization process has produced less value of Ngs.7. In high frequency C-V characteristic measurement curves of the three types p-GaAs MOS structures, hysteresis (AVHIS) with electronic character have been observed. The reason for hysteresis has been attributed to the electrolyte which remain in the pores of the insulating layer. This electrolyte is removed by annealing. But reproducibility in this process is low. For this reason, rather than annealing, an anodic growth without hysteresis is preferred. So far the high frequency C-V curves in anodically oxidized GaAs MOS structures, the hysteresis phenomenon has not been observed. But in this study the high frequency C-V curve for p-GaAs/AS/Al structures obtained under 0.1 mA/cm current density, no hysteresis hâs been observed. This can be explained on the basis of the more volatile nature of the electrolyte used in anodic sulfidization, the formation of Ga2S3 during anodization within the zincblend conforming structure, and the low temperature. Evaluation of the results show that the interface state density (NSSFB) at flat band for p-GaAs/AS/Al structure produced by nonaqueous ammonium sulfide electrolyte at 0.1 mA/cm current density and ambient temperature is found to be 6.5x10 cm`. This shows that this structure can be used for production of circuit elements. The absence of hysteresis in the high frequency C-V curve at 77K is an important development. In addition, in this structure, all other samples at 293K with respect to the magnitude of hysteresis in high frequency C-V curves, have shown lower hysteresis. This structure has achieved the interface state density (Nss=10 cm eV ) which has been sought for many years. The future studies will concentrate on the composition of the in-depth profile of the anodic sulfide layer. Also the behavior of the layer under atmospheric conditions as a function time needs to be investigated. In addition, the reduction of hysteresis with electronic character observed in high frequency C-V curves at room temperatures requires low temperature annealing studies. XI 137