Diodes II: Fabrication by Doping MS&E 362: Materials Lab III Nov. 8

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 2 Topics Impurity doping in semiconductors Doping by diffusion Doping in the liquid phase Other methods of doping: beyond solid solubility Lab procedure and special issues Scheduling lab work this week

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 3 n-type Doping Group IV elements: 4 valence electrons, 4 covalent bonds Add a Group V, like P, As, and Sb: 5 valence electrons, still 4 covalent bonds e-e- 5th valence e- from the Group V atom is weakly bound, so it’s free to move through the crystal, conducting electricity.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 4 p-type Doping Group IV elements: 4 valence electrons, 4 covalent bonds Add a Group III, like B, Ga, or In: 3 valence electrons, still 4 covalent bonds + Group III atom steals an electron from an Ge-Ge bond, leaving a hole – a positive charge – which can move and conduct electricity.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 5 Doping Band Diagram Impurity levels are within k B T of the band edges at room temperature, so they are emptied by thermal activation

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 6 Doping Requires a Solid Solution Two neighboring Group III’s each have 3 bonds and a full shell. Large groups form metallic second- phase inclusions.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 7 Doping by Diffusion Create a composition gradient by putting some dopant on the surface. Heat it up and wait. Dopant atoms diffuse into the host. We will use this method to introduce In to our n-type Ge:P wafers.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 8 Diffusion Doping Pro and Con Pro: –Dirt easy. –First junctions and bipolar transistors were fabricated this way. Con: –exp[-z 2 ] decay of dopant concentration away from the surface. We want a highly doped junction. –Sensitive to surface conditions and unwanted impurities. –Leaves excess dopant on the surface. –Potentially requires a long time and high temperature, which could damage other parts of the device.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 9 Doping in the Liquid Semiconductor single crystals are pulled from a high-temperature melt. Small seed crystal promotes growth and sets orientation. Crystal can be doped by adding dopant to make the melt a solution. Our Ge wafer is already doped with P this way.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 10 Liquid Doping Pros and Cons Pros: –Creates a huge volume of uniformly doped material. –Widely applicable to many materials and dopants. Cons: –Limited in concentration to solid solubility of dopant in the host melt at temperature. –Reduce crystal quality of the host – creates point defects. –Creates only uniformly doped material.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 11 Doping by Implantation: Macroscale Ionize dopant atoms, often from hydrogen precursor gas. Use electric field to accelerate to keV. Slam into the host crystal.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 12 Doping by Implantation: Microscale Nuclear collisions: ion hits a nuclei and transfers energy Electron drag: negative electrons exert a viscous drag force on the positive ion Fast ions come to rest in the host by two processes.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 13 Implant Depth Distribution By changing the ion energy we can control –the depth from the surface of the implanted layer –the implanted layer thickness

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 14 Damage During Implantation  E < E d : the target atoms are not displaced  E ~ E d : simple displacement – creation of an interstitial atom (the original target atom, now displaced) and a vacancy (where the original target atom used to be)  E >> E d : damage cascade (multiple displacements by both ions and atoms) If E d is the energy needed to dislodge an atom, then:

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 15 Amorphization and Regrowth At high enough dose, damage destroys the crystal – renders it amorphous Crystal can be regrown from the crystal underneath in solid-phase epitaxial regrowth

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 16 Implantation Doping Pros Applicable to almost any materials system Clean – very low unintentional impurities Room temperature process Control dopant profiles by implant energy Dope small regions by implanting through a mask Achieve extremely high doping concentration – metastable above solid solubility Industry standard process

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 17 Implantation Doping Cons Creates crystal defects: –vacancies –interstitials –end of range {311} defects –amorphization Still difficult to make extremely thin layers and extremely high concentrations required for future devices. TEM micrograph of {311} end-of-range defects created by B ion implantation into Si and annealing Eaglesham et al. APL 65, 2305 (1994)

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 18 Doping by Thin Film Growth Grow new crystal on top of old in high vacuum. Incorporate dopant atoms in the flux to the surface. Pros: –very high doping levels –ultimate control over layer thickness and depth –very high quality crystal layers Cons: –blanket doping –very best processes (MBE) are slow and require UHV

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 19 This Lab Cleave a small square of n-type, P doped, Ge:P wafer Clean the wafer surfaces carefully. Diffusion dope with In from one side. Sputter gold thin film on the back to make an Ohmic contact. Schematic of the final configuration of our homegrown diodes.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 20 Why Ge? Most microelectronic devices are based on Si. –key is almost perfect native oxide SiO 2 –good insulator, very stable, unbelievably low interface defect density –only etched efficiently by hydroflouric acid We’ll use Ge because of it’s oxide –Oxide layer will prevent good electrical contact to the junction –GeO 2 forms more slowly than SiO 2 and is water soluble.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 21 Why In? Need a Group III element B: melting point 2079  C, oxidizes in air, commercial powders contaminated with C (industry standard is implantation from diborane gas, which is very pure) Al: deep acceptor level makes it a poor donor Ga: melting point 30  C, toxic In: inert, convenient 156  C melting point

P. M. Voyles, U. Wisconsin, Madison, 11/8/ Cleave a Small Square Need a 4  4 mm piece for diode. Divide a single crystal by cleaving: –scribe a line where you want the break –sandwich the wafer between two slides at the scribe line –push down sharply on the overhanging edge with another slide –discard particles / dust

P. M. Voyles, U. Wisconsin, Madison, 11/8/ Cleaning the Surface Start with a simple chemical clean to remove surface organic contaminants. Finish with plasma etch to (slowly) remove surface material, including residual oxide.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 24 Doping with In Heat at 250  C –well above In melting temperature –molten In will dissolve some Ge –when temperature falls, Ge will recrystallize and incorporate some In Heat in vacuum to slow the growth of new oxide.

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 25 Vacuum Furnace Vacuum System High vacuum diffusion pump can only operate between and T –will choke if the top is open to atmosphere –needs another pump to maintain base at less than atmosphere Works together with a rough pump. vacuum chamber gate valve low vacuum mechanical pump high vacuum pump valves A B

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 26 Vacuum Furnace Vacuum System vacuum chamber gate valve low vacuum mechanical pump high vacuum pump valves A B vacuum chamber gate valve low vacuum mechanical pump high vacuum pump valves A B Mode 1: roughing chamber at atmosphere gate and B closed A open Mode 2: high vacuum chamber at high vacuum A closed gate and B open

P. M. Voyles, U. Wisconsin, Madison, 11/8/ Sputter Back Contact Move quickly from step 3 to reduce oxide formation Etch back side of the disk (the other side from the In) a little bit more for cleanliness Sputter ~20 nm of Au on the back side More about sputtering next lab...

P. M. Voyles, U. Wisconsin, Madison, 11/8/11 28 Summary Doping Group IV semiconductors: –Group V for electrons, n-type –Group III for holes, p-type –diffusion, in the liquid, or by implantation Fabricating a junction diode by diffusion: –cleave out a small square –clean the surfaces –dope by diffusion –sputter a back contact. Set the schedule for this coming week.