This might be attributed to reduction of grain boundary scattering by increasing grain sizes

I Think the increasing of temperature lead to increase the pressure inside the chamber , when you try to keep the pressure constant that lead to decrease the number of Ar atoms, subsequently lead to decrease the rate and lower thickness at constant time. 

Due to high substrate temperature, the adatoms migration rate might be very high and the kinetic energy of neutral atoms is very less, so the substrate adhesion is also less as a consequence. Please do some literature survey regarding with Thornton model and kinetic energy which have significant effects on growing thin films. 

Dear colleague Tvarit Patel

Have a nice time and good day.

The films thickness is a function of annealing temperature. The produced materials may be amorphous (short range order materials, SRO) and always characterized  by defects or dangling bonds so it has a random structure  and poor order also it may has many pores. So such these materials have low density and strength. Considering the aforementioned reasons the produced material may has a certain thickness. Because of annealing the unsaturated defects or localized states may gradually annealed out (decreases), producing a large number of structure bonds. The reduction of the number of unsaturated defects reduces the density of localized states and shifts the amorphous to poly crystalline and/or crystalline state and consequently increases the materials' order and decreases the film thickness. So materials of medium and long range order; MRO, LRO  may be introduced. As a resultant, nucleation of different shapes of crystallite size will be formed. The conclusion is thigh substrate temperature will give the chance of small crystallite size to merge with each other to form large size and consequently grain size with different high diameters will be appeared with high quality poly crystalline structure thus will enhance the structural properties and may all crystallite sizes merged together to form poly or single crystals with different shapes of crystallite size characterized by high density and low thickness.

I wish that be useful and help you.

Yours

A. EL-Denglawey

Dear Tvarit

With increasing substrate temperature, thermal energy overcome to adhesion energy and bonding's of deposited atoms on substrate break. for this reason, this atoms desorb from substrate surface and thickness of the films decrease with increasing substrate temperature.

I wish that be useful. 

I agree with A. Eldenglawey that a higher substrate temperature possibly leads to a more crystalline material, and also that this can give a bit higer density. However I doubt if this can account for a more than threefold decrease in film thickness.

Indeed the material that reaches the substrate will have more energy, so also more mobility and can therefore more easilly form ordered crystals. However desorption I would expect only for temperatures above the melting point of the material.

Thank you Pieter, i think for desorbtion don't need to reaching substrate temperature to higher than melting point. Desorption is completely deferent from melting and evaporation. 

Also I agree with first part of your comment but this phenomena can occur in special crystallographic orientation to minimize the lattice energy. 

Ok, interesting, so where does the desorbed material go then? Will it just 'bounce' from the substrate when it reaches it?

They disturb into deposition chamber and can deposit on chamber walls and some of them can deposit on substrate again. 

Good explanation, Thank you all

usually the substrate temperature makes two main functions beside the mechanical support of the film itself; (1) gives the chemicals the energy to reacts and (2) gives the formed particles the energy to move in X-Y plane. the first is not your case since you use physical method. but it still gives the particles the energy to move in the X-Y plane . that will decrease the film thickness and surface roughness values. in the same time it may makes the film more compact. you may try to calculate the density or the atomic packing ability (APE).

i hope that help.